According to the paper (https://www.tandfonline.com/doi/full/10.1080/00822884.2021.1943792 https://www.tandfonline.com/doi/full/10.1080/00822884.2021.1...), it

According to the paper (https://www.tandfonline.com/doi/full/10.1080/00822884.2021.1943792 https://www.tandfonline.com/doi/full/10.1080/00822884.2021.1...), it wasn't widely known (or at least believed) in Italy at the time (since Genoese maps didn't reflect North American lands). There is no reason to suspect this isn't real - it's hard to fabricate a convincing forgery of a medieval Latin manuscript. And this manuscript itself is perfectly plausible - all it's really showing is that one Italian friar somehow (perhaps via Genoese sailors) learnt about what Viking sailors already knew about the New World.