It doesn't follow that your body, when running at a calorific deficit, will continue to run the life support systems at optimal levels. In principle, it's possible (unless you're actually fasting, which is a whole different kettle of fish) for a reduction in energy-in to lead to putting on more weight, if your basal metabolic rate drops more than the food did.

It's complicated. What's true for some people isn't necessarily true for all people. Sure, I'm willing to bet a lot of people – probably most people – are able to choose how fat they are by adjusting their diet and exercise regime, without adverse effects on their health, but that's not a law of biology. I know as many people who struggle to put on weight as struggle to lose it.