For something to be counted as the ability or a part of a character, they have to be able to use it without any previous conditions having been met (previous to comparing one character to the rest). Edo Tensei requires 2 conditions be met prior to the match: that kabuto has the DNA and that the souls are available. Does Kabuto always have the DNA when in his prime? Arguably yes. But will the souls always be available? Well, you could say yes, but then you would have to say "Kabuto is better than madara because he can summon Edo Madara." But then, he can't really, because if madara exists, then he has his own soul, then kabuto can't summon edo madara. Which means we have established the condition that it requires the soul to have been dead and not sealed away. But in comparing a list of people from all times and statuses, you must only use things that can work in every scenario. You can't just say "Kabuto can only not summon Madara when he is fighting madara" Because if it is true in that scenario that he can't summon madara, if it is true in that scenario that madara is alive, what stops it from being true in every other match up between him and another character?
If the condition of Madara being alive stops that sect of the edo tensei from working, then every time you list his ability to summon madara as a part of his abilities, you are assuming that he is already dead. And while, for Prime kabuto this is true, let's say you are looking at prime Kabuto and comparing him to prime Hashi. In the perspective of Prime Hashi, Madara is alive. In comparing 2 characters, when they can be from any time period, time becomes a variable. And when you have to take into account every single time that has ever happened, you will see that summoning madara is not valid over every single one of those times. Which means his ability to summon madara is only sometimes true. And where time is always X, then it is not true that Kabuto can summon madara, because for something to be true it must happen 10 times out of 10.
Kabuto's ability to summon Madara is not a true ability, because it is dependent on certain conditions having been previously met. Because it is not fair to make assumptions based upon what would be most favorable for a certain character, you can only factor things in as being true or false, because if something is sometimes true then saying whether it is the case in this scenario or not is an assumption, and just as you can't assume that Shikamaru has set traps before a battle, you can't assume that certain people are dead or alive before a battle (where you are taking people from every single time period and so time is a variable). Therefore, you can't use edo tensei as one's own ability.
Oro can be put in place of Kabuto and any ninja whatsoever can be put in the place of Madara in this argument, the same will still apply.
I might as well add that this "paradox" does not apply to let's say EMS users. I will keep this brief. Let's say that Kabuto goes back in time to fight Hashirama prime. Although the DNA that kabuto has is the same, the souls that are out of bodies or that even exist change, because while kabuto can collect DNA and move it out of the system of time and into the system of himself, he cannot collect souls, he can only use the souls he has access to. And depending on what time period that Kabuto is in, the available souls will change. Because the available souls are different depending on what time period Prime kabuto is being looked at in (Again, time must encompass all time, as characters from all of time are considered), Kabuto's edo tensei only works sometimes, where Time is X. Therefore it is not a true ability, therefore you can't factor it in. However, if you were to send an EMS character back into, let's say EMS sasuke back to itachi's prime, just as Kabuto already has possession of his DNA of people (and as any character would have possession of their weapons and such), EMS sasuke already has Itachi's eyes in him, separate from the system of time. No matter what time period it is, EMS sasuke will always have EMS.
That is the key distinction to be made, that when you take someone from their prime and compare them to others, the necessary assumption that you must make is that they will have the abilities of their prime time. But Kabuto's ability is not to summon madara. That is not the ability of the edo tensei. His ability is to summon people whose dna he has and whose souls are available. Unless Prime Kabuto will always have Madara's soul, which he doesn't unless he has a secret soul capturing device that we've never heard of, it is not Prime Kabuto's ability to be able to edo tensei Madara. And where time is X, and so every possible time period is factored in, it is not true in the system of time or in the system of Prime Kabuto that he can summon Madara, so it is not a truth in comparing Kabuto's abilities with others. And, again, you can sub in "Madara" for any other character and the same will hold true.
If the condition of Madara being alive stops that sect of the edo tensei from working, then every time you list his ability to summon madara as a part of his abilities, you are assuming that he is already dead. And while, for Prime kabuto this is true, let's say you are looking at prime Kabuto and comparing him to prime Hashi. In the perspective of Prime Hashi, Madara is alive. In comparing 2 characters, when they can be from any time period, time becomes a variable. And when you have to take into account every single time that has ever happened, you will see that summoning madara is not valid over every single one of those times. Which means his ability to summon madara is only sometimes true. And where time is always X, then it is not true that Kabuto can summon madara, because for something to be true it must happen 10 times out of 10.
Kabuto's ability to summon Madara is not a true ability, because it is dependent on certain conditions having been previously met. Because it is not fair to make assumptions based upon what would be most favorable for a certain character, you can only factor things in as being true or false, because if something is sometimes true then saying whether it is the case in this scenario or not is an assumption, and just as you can't assume that Shikamaru has set traps before a battle, you can't assume that certain people are dead or alive before a battle (where you are taking people from every single time period and so time is a variable). Therefore, you can't use edo tensei as one's own ability.
Oro can be put in place of Kabuto and any ninja whatsoever can be put in the place of Madara in this argument, the same will still apply.
I might as well add that this "paradox" does not apply to let's say EMS users. I will keep this brief. Let's say that Kabuto goes back in time to fight Hashirama prime. Although the DNA that kabuto has is the same, the souls that are out of bodies or that even exist change, because while kabuto can collect DNA and move it out of the system of time and into the system of himself, he cannot collect souls, he can only use the souls he has access to. And depending on what time period that Kabuto is in, the available souls will change. Because the available souls are different depending on what time period Prime kabuto is being looked at in (Again, time must encompass all time, as characters from all of time are considered), Kabuto's edo tensei only works sometimes, where Time is X. Therefore it is not a true ability, therefore you can't factor it in. However, if you were to send an EMS character back into, let's say EMS sasuke back to itachi's prime, just as Kabuto already has possession of his DNA of people (and as any character would have possession of their weapons and such), EMS sasuke already has Itachi's eyes in him, separate from the system of time. No matter what time period it is, EMS sasuke will always have EMS.
That is the key distinction to be made, that when you take someone from their prime and compare them to others, the necessary assumption that you must make is that they will have the abilities of their prime time. But Kabuto's ability is not to summon madara. That is not the ability of the edo tensei. His ability is to summon people whose dna he has and whose souls are available. Unless Prime Kabuto will always have Madara's soul, which he doesn't unless he has a secret soul capturing device that we've never heard of, it is not Prime Kabuto's ability to be able to edo tensei Madara. And where time is X, and so every possible time period is factored in, it is not true in the system of time or in the system of Prime Kabuto that he can summon Madara, so it is not a truth in comparing Kabuto's abilities with others. And, again, you can sub in "Madara" for any other character and the same will hold true.