In the same vein as much of the rest of what you're saying, the other thing that I feel like people always neglect with their "Calories in/calories out" and "Bodies can't violate thermodynamics" is that the human body can adjust how efficiently it processes food, colloquially known as a "slow" or "fast" metabolism.

While it's true that the human body has no answer to a true calorie deficit (except the incredibly powerful and effective one of tweaking satiety and hunger signals), as long as you're eating more calories than you're strictly burning, your body can simply take longer or less time to digest the food you put into it and extract more or less energy from what you're eating, which can make an enormous difference without you changing your intake at all. Which means that people can absolutely eat identically, have identical appetite levels, and have extremely different body types.

If you're exercising more, sure, your body will make you more hungry, but it will also work harder to squeeze every possible calorie out of what you're already eating. If you cut down on what you eat, your body will work even harder at it, to the point that you could literally eat less, work out more, feel hungry and tired all the time, while getting fatter, because your body is worried that you're in a famine and in a physically stressful environment and is desperately trying to signal to you to conserve as much energy as possible, eat as much as possible when you find food, and at the same time trying its best to make the most of the food you give it.

And at the same time, someone else's body might simply not do that. It's crazy!