It is. If you think it isn't then you're missing something (like maybe calories excreted unmetabolized?) But it's also a bit of a reductio ad absurdum and isn't

It is. If you think it isn't then you're missing something (like maybe calories excreted unmetabolized?) But it's also a bit of a reductio ad absurdum and isn't a very interesting take on the problem (and I say this as someone who also used to spout it.) Within a pretty tight margin, calories in minus calories out DOES predict weight gain. The real question is why someone feels the need to eat more calories than they burn even when they already have ample stores of calories, and what they or others can do to change this. And THAT is a much more interesting question.