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.
