> Calories in calories out is not an immutable law.

It actually can be, if you define both "in" and "out" in a way which makes it so; but for any definition of those terms where it is an immutable law, we, in practice, have only very rough proxy measures for at least one, and maybe both, of "calories in" or "calories out", and the problem then becomes mistaking the proxy for the actual figure of merit.

But the bigger problem is that CICO is only a starting point, it a set of targets, and an actionable plan that works in the real world isn't just the targets the plan is directed at.