If they are a historian something like this would take a mystical person like Leonardo even more mythological. It's almost folk-lore levels of impressive the origin of the greatest minds of western history would be from a less traditional background.
If I was a historian I too would be skeptical. It's just almost too good to be true as far as someone who historically is often portrayed as a type of hero to humanity due to his artistic and objective contributions. One of the greatest painters in history was also born of a slave mother emancipated by that great and inspiring painter's father? It's even romantic!
No one said it was impossible, just impossible for someone in a field of study to believe outright. Good on them for going out and finding out the truth for themselves. Id say that's what it's all about.