If people before 1970 eaten approximately their TDEE (2000 calories?), than a 20% increase in calories eaten means something like 400 calories over TDEE per day - every day, all your life. Of course people get fat like this.

This issue caught my eye as well, and I find the argumentation quite sloppy in some parts. Also see my other post https://news.ycombinator.com/item?id=27808936

Now take several of these "insignificant" factors, add interaction effects, and we potentially get a much richer picture.