Let me preface this by saying I read the book over 10-years ago, and I don't have a background in science, so some of the book was lost on me - but...
They do eat their share of bugs, but for some reason, the bugs they have access to, and in the numbers that they have access to those bugs, doesn't quite give them the nutrition they need. The book drew some fire from some of the vegans in the world, because it essentially says that absent quality meat & fish protein, brain development can be compromised. Like I said, it's a bit controversial.
I don't find that controversial. I guess a high protein BBQ-based diet reduces anxiety too.
It’s not just the protein. Vitamin B12 is essential for proper brain function; the first thing my doc did when I complained about insomnia and anxiety was check my B12 level. The best sources of B12 are meat and dairy- a vegan diet without supplements will lead to deficiency.