Yes sure calories in calories out. But metabolism has so many variables. It can be revved up and burn a lot of calories. It can become quiescent when you are in starvation mode. What causes it to start using the fat reserves is definitely more than a univariate analysis. If there is a catalyst that starts breaking down fats faster it won’t matter that you are eating more.
Some people are naturally skinny and others put on weight quickly. This is already well known and shows that there are complex genetic inputs as well as other ones. We have to discover what those are. So saying it’s just calories in/out, stupid... nope!