I am baffled by this explanation. Physical movement is one of the things we have lots of and lots of data about. We can even measure it reasonably accurately us

I am baffled by this explanation. Physical movement is one of the things we have lots of and lots of data about. We can even measure it reasonably accurately using devices that most people always carry in our pockets. If the solution to the obesity epidemic was as simple as "walk around more" then we'd have already solved it. But while there are huge benefits to going from zero walking to "some walking", those benefits don't scale to the levels needed by simply adding more steps. TL;DR If adding a couple miles per day of walking would resolve the obesity epidemic, it'd already be resolved across a huge swath of the population. But all evidence is that the marginal weight-loss benefit of walking tails off rapidly, unless it's paired with a restricted-calorie diet (which is exactly the point: our 1960s parents/grandparents didn't need that.)