> If they don't eat as much due to higher satiety, then you are not replacing with equivalent calories.

That's one factor of why eating fat instead of carbs defeat the calories-in-calories-out idea. But it's not the only one. Both the lack of release of insulin and ketosis helps the body work much better.

> 5 calories of one really will have the same effect on your waistline as 5 of another, holding all else equal.

That's the problem, nothing else is equal when you have a vastly different proportion of macronutrients.