I was similarly skeptical until I read it (ever since the first part appeared on HN) and it convinced me on many points.

That said, I think you're half right? SMTM's environmental contagion hypothesis is an explanation for population-level obesity rates, not individual cases of obesity. Anybody who cares sufficiently about their own obesity can change it through individual efforts. However, on a population level, "just change your habits", or even worse, shaming people who are obese, is a less effective strategy than "figure out what is causing their bodies to signal hunger and store fat in dysfunctional, self-harmful ways that they didn't used to anywhere on earth until the mid-1900s".