Earlier quoted context omitted.
At the minimum, a lack of physical activity has to be included as a factor in this along side diet.
Haven't you heard? The new wisdom is that exercise makes you even fatter. Well, I exaggerate, but I'm referring to a recent Time story saying 'exercise won't make you thin' (the idea being that you still need to eat less or you'll just get hungrier and never lose weight). You are quite right, but 'pin the tail on the donkey' seems to sell better :-/
We are just going back to what we knew 40 years ago.
Basically, between 1960 and 2009 we thought exercise will make you thinner.
Outside those times exercise was thought to increase appetite.