A) It's anthropology. There is nothing concrete available; the entire field is an attempt to learn things by investigating human societies. In this case, the allegation is that a specific tribe have enough stories about human-like but non-human creatures in their region to be taken seriously (in conjunction with the established presence of non-human hominins in that region in the fossil record) as a record of recent history.

B) The article is promotion for a book, which I would hope would have a more cohesive presentation of evidence.