Physics is pretty adamant it is.

Physics is even more adamant that mass-in-mass-out is the determinant of weight balance. Try drinking less water, which is very massy, and eating lighter foods. I hear fat has more calories per gram than protein or carbs, so if you take your calories by fat you can't help but lose weight! Right? If you were previously taking in most of your calories as protein and carbs, changing all your calories to fat can't help but nearly halve your mass-in. As long as you don't inhale more air to make up for it, and keep breathing out and sweating and excreting the same amount, physics says you literally can't help but lose mass! Literally every time someone keeps strictly to the MIMO diet, they lose mass - period. Every time someone gains mass, strict examination shows that they took in more mass than the MIMO diet says they should, given how much they were excreting, exhaling, and sweating. If you keep to the MIMO diet and still have problems with your weight, it's probably because you're not staying in the same gravity and have moved to a heavier planet. If you can't keep to the MIMO diet, you just need more willpower to avoid taking that extra inhalation.

Modulo caloric loss by e.g. ketones in urine, physics says CICO has to be 'true in some sense', but it doesn't say CICO has to be any more useful than MIMO.