If someone ate the same three meals for 20 years and maintained a consistent weight, and then due to some physiological change started to put on weight eating the same amount, I'd find it reasonable but perhaps incorrect to attribute their weight gain to overeating. But yeah, technically at the end of the day, they could eat less :)

Yeah, it's not very useful in practice of course. Telling someone "eat less" has very unimpressive real world impact