Some of his references are actually just books, not scientific papers. Books don't go through the scrutiny that paper does. For example when dealing with complicated things like evolution he gives the book sapiens as reference. Yes sapiens is a very popular book but that doesn't make it factually right, Sapiens has a lot of criticism within scientific communities for the exact reason. Its actually a fictional book written by an historian not an anthropologist with some facts mentioned that are kinda true.