I'm not really seeing proof to the counter. Let me try to make my claim clearer.
Take humans. Most historians would agree that at some point Cro-Magnons interbred with Neanderthals. Assuming this, technically all modern day humans have traces of Neanderthal DNA in them. Does it make us a Neanderthal? No, but technically would could be partially classified that way. Even take the prior, Cro-Magnons. We are not technically considered that anymore, but its rooted in our DNA and thus we could still technically be classified in that way, us being their decedents at least. Point is, Uzumaki is to Senju as, perhaps, modern Humans are to Cro-Magnons/ Neanderthals.
My argument is that Uzumaki share DNA with Senju, meaning that the 2 are not completely separated.
OK , I just wanted to point that Uzumaki being descendants of Ashura is not a fact.
You know that monkeys share DNA with you?
Your argument is true , but it is also true for monkeys , Uzumaki share DNA with monkeys , and?
I think we can believe Kushina when she says that Uzumaki and Senju are
distant cousins (it's a relative word , even you are a distant DNA relative of myself).
So , I agree that they share DNA.
But I don't agree to the fact that Kushina (or every Uzumaki) is a descendant of Ashura , cause she can share DNA with him without being his descendant , like you and monkeys.
Your exemple paralleling Uzumaki with modern humans (sapiens sapiens) and Senju with Neanderthals doesn't work great , it is more complicated than the subject itself.
To prove that something is untrue , you just have to prove that something else is possible.
The fact that Kushina could be a descendant of another guy than Ashura , is enough to disprove that she is a descendant of Ashura.
So if you don't see the counter , look better.