[William Cloud Hicklin]

That last, from the Shibboleth, is a powerful argument; perhaps it left an impression in my mind without my consciously remembering it.

It certainly reinforces the idea that works like the QS and the Annals were Numenorean works, either saved by Elendil or recorded from oral tradition in Arnor (an additional headcanon assumption is that Arnor's scholars sent copies of their most important books to Elrond for safekeeping)

But it doesn't get around the problem that, in Numenor and Arnor both, scholars and scribes would have had enough interaction with the High-Elves to know the Truth; flat-world versions can't be explained away like RW creation myths, made up in ignorance by primitives. While possibly the Peoples of Beor, Marach and Haleth might have believed in a flat world on their journey to Beleriand, talking with individuals who had spoken personally to the Valar would have quickly disabused them!