Interesting that they talk about their predecessors completely dying out. There was a fascinating Time Team special where they mentioned that Britain has had seven completely separate waves of inhabitants - with the predecessors apparently all dying out.

Probably a coincidence though as the timescales won't match up but fascinating to even consider that there was some historical truth behind those old myths.

Sounds similar to the stories of the deluge in many cultures, that supposedly wiped out all or most earlier life, once or periodically. Bible's Noah's ark, Hinduism's pralaya (IIRC, primeval flood), many others ...

If there really was such a thing, it might explain a lot of these stories.