I think the problem with you being so glib about this is that the "common sense [...] diet and exercise" shtick has been treated as the last word. This pokes a major hole in that orthodoxy. And that orthodoxy is already pretty tattered.

That's important in this context because many people use the orthodox view as a club: if you're fat, then obviously you're a bad person, a lazy glutton who can't be bothered to take some simple actions.