Appetite differences are almost certainly the primary factor in why some people get heavy and others don't, but that isn't inconsistent with CICO at all.
Successful dieting is about managing the sensation of hunger. That’s it.
Yet, some people have a different appetite, or a different reward system wiring for food, and they just don't feel the same impulse to do it even when they're stressed out.
All of these are valid approaches and can work! But you have to first know that CICO is what matters, in order to appreciate your (very correct) statement that "Successful dieting is about managing the sensation of hunger."
If someone thinks that IF works regardless of CICO, they're going to get it wrong. I know, I was there many years ago before starting to count calories and losing a lot of weight. (Though counting calories works for me, I don't necessarily advocate it for everyone!)
One other small point: While managing the sensation of hunger is a big part of it, other people "overeat" for other reasons. E.g. I have far less issues with hunger, than I have with just liking novelty in foods - I like to try a lot of different things, and lately I've started to cook a lot. That is a bigger source of problems for my dieting goals than feeling hungry, personally. Other people eat for stress, or to stay awake, etc. Hunger is a big part of the equation, but not the only part.
In my search for a diet that would allow me to avoid gaining weight, the main successes were achieved by replacing various kinds of food that were tasty, but which did not satiate me before eating huge amounts of them, with other kinds of food that are also tasty, but which satiate me after eating a moderate amount of them.
Some instances this seemed due to an incredibly fast metabolism, others were due to GERD/IBS or some other factor that limits the craving.
The differences are real, even if it’s just something like gut biome.
My sister and I grew up with the same diet. We didn't starve, but neither of us had lots of access to junk food. Yet she could be hungry once she got home and after half a plate to be already satiated.
Today she has tons of junk food in her house yet she doesn't feel the compulsion to eat more than a few bytes. I'm down to 71 kg from 83 kg back in June, but I still can't have a single cookie package in my house.
I have said "a diet to avoid gaining weight", not "a diet that allows gaining weight".
If I eat more than 1900 ... 2000 kcal per day, I gain weight rapidly, so the problem was reducing the amount of energy intake so much, while ensuring an adequate daily intake of proteins and of any other essential nutrients, and without feeling that I am starving.
I agree that there are differences between people and there are also differences for a single human depending on circumstances, e.g. in many cases when you are ill you lose your appetite.
Nevertheless, for the same human, different foods containing the same amount of energy can behave extremely differently from the point of view of causing satiety, both immediately after the meal and some hours later.
For remaining satiated many hours after the meal, at equal energy content food with higher content of proteins and/or fat and/or fiber is more effective.
For being satiated immediately after the meal, at equal nutrient composition food that is more consistent and requires prolonged chewing causes satiety much faster than soft food such as creams, mashed potatoes, ground meat and the like.