[Mithadan]

In Tolkien's later work, he struggled a bit with the distinction between legends of the Elves and legends of Men. A direct reference is found in the introduction to Tom Bombadil where he attributes the various poems to different sources and states that the Tale of Turin was written by a man.

In later HoME volumes, this distinction arises again where he attempts to address or reconcile the "flat earth" view of Arda with the "round earth" view by suggesting that the flat earth view as well as tales in the Silmarillion as well as the Ainulindale may have been mannish versions of legends originally told by Elves and either "corrupted" or changed in the retelling over many years or simply misunderstood by the Numenoreans. JRRT ultimately either rejected this view or simply never decided how to deal with it.

The Red Book presumably contained Bilbo's versions of Elvish tales heard directly from the Elves, but may have been recorded inaccurately due to lack of complete understanding. Further, the Red Book was supplemented by scholars in Gondor who may have included remote rumors and legends (true or untrue) as well as "facts". So when LoTR and the Sil. say "it was later said" or "it was sung", this may refer to "mannish" legends rather than true Elvish tales.

Elendur also makes a good point that the Elves may (wisely) have not told the Numenoreans about Tuor being given the fate of the Elves for the reasons he stated. Man-of-the-Wold, the significance of the 50 years is that Tuor, if he arrived in Valinor at all, would have been "elevated" to Elven status by the time the Host of the West came to Beleriand. If he were somehow given immortality this would be a significant event and the tale would have been related accurately to Elves and/or men and thus would not be a rumor or legend but rather treated as a "fact".