[uup]

I think the author's fundamental misunderstanding is that most people, even people like himself who "take learning seriously" don't read popular non-fiction to learn. Most people read for entertainment. So you shouldn't be evaluating how much one learned from Guns, Germs, and Steel, rather, you should compare how much you got from Guns, Germs, and Steel compared to a TV show like Breaking Bad. So, to answer the author's question: what should be done about it? Nothing.

The reasons why textbooks work is because you don't just read a textbook. You go over the same information repetitively via exercises. And it's natural that a course is even more effective than a textbook. You can never ask a textbook a question for clarification like you can an instructor. That doesn't mean the textbook is bad, it just means that that a textbook + an instructor is better.