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.
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.