If all of humanity were wiped out except for Noah and his family, then there shouldn't be any Flood traditions from any other cultures. Only Noah's story should survive. The fact that there are hundreds of such tales, each of which somehow survived The Big One spoken of in everybody else's tale, indicates that none of them were global.
That's a good argument. I wish I'd thought of it!
Further, the "historical Noah tale" believers appear to mostly be unaware that it is a distorted version of an earlier Middle Eastern fable, the "Epic of Gilgamesh", reworked as a morality story of divine retribution for the sins of men. About the only people who don't accept that the Noah story is derived from the Epic of Gilgamesh are Christian Young Earth Creationists. Everyone else, believer or not, accepts the mainstream dating of the two stories.