There is also a legend of incestuous royals connected to the Boyne valley tombs in Ireland, written down in the 12th century. DNA examination of a person buried in the tombs found that his parents were indeed very closely related. This suggests almost 5000 years of oral tradition, even surviving a fairly big cultural replacement event.

If that can happen, it's not so surprising a story can last 20-30000 years either - especially not if there's not much else making a similar impression to tell stories about.