That the relationship of calories in to calories absorbed is linear needs some heavy proof before I'll believe it.
My favorite counter point to calories in/out crowd is that the body is a dynamic system. The calories out portion doesn't stay constant. I have lost weight while eating more because my base metabolism increased. In "That Sugar Film," the guy eats the same amount of calories and same activity level but starts to include "daily recommended sugar" and increased his weight. The body is a dynamic system and will change how it stores and processes energy. Yes, you can't violate thermodynamics and eating a calorie deficit will cause weight loss (and possibly other issues), but the system tries to adapt.
(a) CICO (calories-in/calories out) is not the same as the energy conservation principle, you can claim that CICO model does not work while firmly believing in the energy conservation principle
(b) CICO isn't untrue, its just too simple to model the bodies approach to managing the fat stores. Fat storage is very much controlled by insulin and cortisol. If one or both are high, your body is primed to increase your fat storage
CICO crowd equates 7000kcal with 1kg of fat body mass and then you can do calculations that losing 10k of body mass in a year can be achieved by reducing caloric intake by 200kcal/day, or increasing expenditure by the same amount, or a mix of both.
Experienced dieters know that this doesn't work as easily.
I was trying to loose weight with CICO and hang out in the CICO-crowd forums for 2-3 years until I realised I couldn't lose more than 10kg that way until I plateaued. And in these forums, it seemed to be similar for many.
Then, with fasting and/or keto forums I hang out in there are so many more credible reports of extreme weight loss. And that is because water fasting and/or keto reduce the amount of insulin your body emits, and thus allows your body to really tap into the fat reserves it has in store.
When you weigh a certain weight, you have a certain resting energy expenditure.
Let's say the average 200lb(Sorry for the American units) man burns 2000 calories a day. If he then goes a week only eating 1500 calories, he will have burned a pound, and now only weighs 199lb, and his new resting energy expenditure will be slightly less. The next week when he is still eating 1500 calories, he will lose only 0.99 pounds.
So of course you will run into a time eventually when you plateau. At some point 1500 calories a day will be your new equilibrium weight and you will no longer lose weight without additional changes.
This is typically an argument the other way, as when you do the math this way it's very clear that the energy when stored as fat is incredibly energy efficient. An extra 100 calories might mean that you gain a pound in slightly more than a month, but you won't settle on your new equilibrium until you gain more than 10 pounds. According to wolfram alpha, the basal metabolic rate of a 200lb male is 1997 calories. The basal metabolic rate of a 300lb male is 2465 calories. The daily energy expenditure of a 200 pound male, who runs daily for 3 miles over 30 minutes, is equivalent to a 300 pound male. 100 pounds of fat is actually a shockingly small amount of daily calories difference.
Anecdotal evidence: I've lost 70 pounds, then regained the 60 pounds, then lost the 60 pounds, and am currently up 30 pounds. Simple does not mean easy.
The more you eat the less your body will absorb, as your digestive system will digest and absorb more of a 100Kcal meal that a 300Kcal meal. Also the more you consume higher your metabolic rate will be, while the converse it also true. Because of that you you have to restrict your calorie intake by a fair amount to lose weight, and overeat by so much to start gaining weight. Don't forget that all calories are not created equally, as some foods are digested better that others, and different combinations of fat, protein and carbohydrates will affect digestion. The addition of simple carbohydrates raise insulin which can also increase absorption, which is great for gaining weight and stimulating your appetite.
Calories out is equally suspect. Your body has mechanisms to adjust his many calories you burn, and those are not under conscious control.
You get fat one more morsel of food, and one less body movement at a time. You lose weight the same way. One less morsel of food, and one more body movement at a time.