I had a professor in college that had a theory about these types of people. This sort of behavior wasn't as common in the past, pre-Internet days, since when someone came out with these stupid HAES ideas, they got shot down pretty quick. It got nipped in the bud.  Today, someone goes online and finds an obscure corner of the Internet that shares their ideas. They continue to delude themselves and others to the point where it isn't an idea, it's a fact. That genetics is why they, and so many others, are fat. That echo chamber makes it easy to ignore clear, logical evidence to the contrary, like the amount they eat, the infrequency of their exercise and the fact that this wasn't an epidemic 20 years ago (those genetics sure came out of nowhere!).  Now suddenly that little obscure corner of the internet isn't so little or obscure anymore.  He wasn't specifically talking about fat people, but the concept remains the same.