> But, the discussion of "primitive cultures" is so problematic that it undermines my ability to take him seriously.

Consider then that maybe it's a flaw in your ability to #1 - "to distinguish “the things my brain tells me” from “reality”". Especially that there's little "discussion" of "primitive cultures" there, just a list of factual observations about them made by various people.

What is the problem with saying that cultures with differing levels of technological and social complexity present different sets of cognitive challenges to their members? If the word "primitive" bothers you, substitute "lower levels of technological complexity" to get the same endpoint.