No, you actually do have a lot of leeway in how much you eat, where you body stays the same weight, because the body adjusts how much energy it uses. Also "excess calories gets you fat" might be a circular argument or truism or whatever, as in excess calories is excess only because it gets you fat.
So there’s no way to gain or lose weight. I see
Lol. I was actually debating whether to add something like “Obviously, once you get past a certain point, you start gaining or losing weight”. I figured it would be obvious and I wouldn’t have to say it but apparently not.
It's non-obvious because it completely undermines your point
It does not. Either you didn’t read my post or you don’t know what leeway means. The idea is your body tries to keep you at the same weight by adjusting its calorie usage. Not a complicated concept. And not a complicated concept to understand that the body can only adjust so far before being forced to put on weight or lose weight.
No one eats excess calories as any type of supplement or natural enhancement, so your example does not work as a counterpoint to the free lunch principle.