It's not exactly a "record" of the event though, is it? It is a story of a flood, borrowing heavily from the Epic of Gilgamesh. Sure, both Christians and Muslims believe it because they use many of the same texts, both being Abrahamic religions. Whether it has any basis in actual events is not easy to ascertain, but it is pretty much clear from writings in other places that the idea of a worldwide flood is not at all universal. I do not doubt that there were large floods in history. Certainly none covered the whole earth. That much is easy to prove. But to an uninformed scribe who probably never travelled more than a few hundred miles from home in their whole life, "all the earth" is a very relative term.
I give them a pass for being ignorant. It wasn't their fault. I don't give a pass to people who believe, without skepticism, the writings of ignorant people many centuries ago. Modern people have no excuse for such ignorance.
To call it the early book of Abrahamic religions "historical" is a great and unjustifiable leap of faith. "Legendary" is a much better fit.