Written accounts are still vastly superior to oral tradition of course, their accuracy is on another level. But that doesn't mean there is absolutely nothing to glimpse from old myths.
Because they're cherry-picked examples fished out of a sea of nonsense. You can't ignore that the body of oral tradition is almost entirely florid fiction, and claim that a few bits and pieces that vaguely resemble reality are evidence that oral tradition preserves information over long timescales. It's methodologically invalid. That kind of analysis gives the same result ("we found an ancient myth that resembles a fact"), independent of whether the proposition, "oral tradition preserves information", is true or false.
It's a classic fallacy.
https://en.wikipedia.org/wiki/Cherry_picking ("Cherry picking, suppressing evidence, or the fallacy of incomplete evidence is the act of pointing to individual cases or data that seem to confirm a particular position while ignoring a significant portion of related and similar cases or data that may contradict that position.")