The normal claim is a bit weaker, that under caloric restriction the body situationally reacts by doing a lot less work. You also see reports of reduced cognitive abilities and whatnot, consistent with lower energy expenditures.
I think those reports are overblown, especially with little mismeasurements on the order of 1Tbsp of butter or the fact that many foods have labeled calories 20% less than the actual caloric content, all of that combined with thinking that exercise is more impactful than it actually is (especially weight training), but the core claims aren't totally ludicrous at face value.
I don't have the reference study on hand, but I believe the brain requires approximately 600 calories a day, or 150g of carbs. Switching over to use ketones requires going into ketosis, which most people never do.
When you look at how our hormones are affected by different types of macronutrients, there are dietary ways to restrict calories, prevent hunger from becoming an issue, and still maintain a decent level of mental acuity.
I've found that aiming for 150g of carbs a day and .7-1g protein/pound of body weight to be fairly self-regulating. I have to eat more carbs on days I do BJJ (pretty glycogenic sport, but, admittedly, has become less demanding the closer I got to my black belt), but using that target as a rule of thumb has worked out well for me over the years.
When we talk about CICO, there is the reality of our hormones that we have to consider--at least in terms of human behavior. There are ways to diet with ease and no discomfort, but yo-yoing between binging on snacks and trying to not eat anything isn't very feasible.
I think those reports are overblown, especially with little mismeasurements on the order of 1Tbsp of butter or the fact that many foods have labeled calories 20% less than the actual caloric content, all of that combined with thinking that exercise is more impactful than it actually is (especially weight training), but the core claims aren't totally ludicrous at face value.