We have many things in hand. The Topkapi Bible is written after Muhammed, that is a fact. And, none of the earlier Bibles mention Muhammed.
Isn't it "curiously" convenient that, after Muhammed appeared on the world stage, there appears a Bible that mentions him explicitly?
Bottom line is, like many people before you, and many after you, you choose the explanation most fitting to your preexisting belief set, essentially singling out the weakest piece of evidence and proclaiming it to be the right one.
rumborak