Well he was unlettered, he could not read or write nor was there any scholars. Nor is there any evidence to suggest he did that.
Because if he learned from someone else then maybe he can hide it for a while but not for long and he would eventually be found out. There are no evidence to back this even from his enemies’ of the time this was never part of their insults.
Also would you criticise yourself in the middle of your book, He turned and frowned sura 80.
Also the only bible available was not whole it was in parts and it was in Hebrew. He spoke Arabic so he would've had to learn another language this also would've noticed by someone.
So this is an old argument and it holds no water.