Hard to take this seriously when the author believes the Yanomami represent some kind of pristine culture, untouched by civilization. Instead they are more like refugees -- survivors of smallpox and other European diseases, and also of European colonizers. A few hundred years ago, they moved to some of the least hospitable land on the continent and were forgotten by the outside world.
They are unique, certainly, but hardly representative of prehistoric humanity.
Are you arguing that they were once a lot more sophisticated and then regressed back to the stone age? It seems somehow unlikely given the description. Even if they fled contact with outsiders, they wouldn't have forgotten everything along the way.
Certainly the destruction of their actual civilization, and moving to a new biome with different resources would have entailed the loss of much technology. But I'm mainly saying that the author isn't even aware of their history, and treats them as a stand-in for prehistoric peoples everywhere.
They are unique, certainly, but hardly representative of prehistoric humanity.