People should really stop repeating this sort of
cows are spherical BS without any understanding of the actual science. That book I mentioned earlier has a decent review of the current state of the art, in layman's terms. The earlier book by the same authors is perhaps even more comprehensive, but is less focused on exercise metabolism specifically.
Maintaining a caloric deficit is clearly an important part of losing weight. However, that observation is about as useful as saying that drowning people should simply focus more on breathing air instead of water.
Here's a video that pretty thoroughly debunks the
calories in = calories out myth. In particular, the rat study mentioned around 6 minutes in shows an increase in body fat from relatively fewer calories in, which is the opposite of what the myth predicts. I see no reason to believe that humans are somehow metabolically simpler than rats.
https://www.youtube.com/watch?v=TSt8A4Iarts
TLDR: metabolism is complicated and this kind of over-simplification helps nobody.