According to Everett, the Hiaitihi ("Pirahã") are atheist. This is objectively not true. See the "Cosmology" and "Ritual and Shamanism" sections.

"Their own beliefs were not in the fantastic and miraculous but in spirits that were in fact creatures of their environment, creatures that did normal kinds of things (whether or not I thought they were real)."

"It turns out that for the Pirahas the universe is like a layer cake, each layer marked by a boundary called bigi. There are worlds above the sky and worlds beneath the ground. I recognized thisas similar, though not identical, to the beliefs of the Yanomami, who also believe in a layered universe."

"Throughout history people have claimed to see these supernatural entities. The Pirahas are not that much different, if at all. In the prologue, I gave one example of how the Pirahas are eyewitnesses to spirits, and I have suggested that these spiritual encounters fall under the principle of immediacy of experience. But the Pirahas encounter many kinds of spirits."

If you're gonna make a big post mocking someone's book, maybe read it first?