I'd say there are some interpretation errors here.
Obito said that if Itachi knew the truth about him then he would be dead. That statement could mean anything. It doesn't necessarily automatically mean "Itachi would beat me in a straight up fight to the death with no intel, prep time, assistance, etc...". For all we know, Obito could have meant that Itachi would have prepared in advance a trap that could kill Obito (kind of like how Konan with prep time set up those 600 billion explosive tags for the purpose of killing Obito). In fact, this is probably what Obito really did mean, seeing as how Itachi did actually set up a trap meant to kill Obito (implanting amaterasu in Sasuke's eye).
In short, the bold statement is false. Obito never said that. Like I said above, there are many reasons as to why Itachi knowing the truth could have led to Obito's death.
Another possible reason for example (aside from the one I already listed) could be: If Itachi knew the full truth about Obito (his identity, abilities, etc...) then perhaps he might have leaked info to Konoha (and specifically the info that Kakashi's eye would be key in countering Obito's kamui), and then that could have led to Obito's death at the hands of Konoha.
The bottom line is:
The statement "If Itachi knew everything about me, then I would be dead", does not equate to "Itachi was stronger than me and could beat me in a straight up 1 vs 1 battle".
Also, Itachi never "kept Obito at bay" as you say, at least not in battle. In fact, Itachi and Obito never even fought.
Anyway, this thread wasn't about who is stronger as an individual. It was just about whose mangekyou sharingan was better/worse in general (mainly about the techniques and a mixture of factoring in their practicality in day to day life, their battle power, and whether or not the Uchiha in question had to suffer from harmful effects).