I think what they mean is "they eat whatever they want and however much they want" and still don't get fat. Some people just only want to eat an amount that do

I think what they mean is "they eat whatever they want and however much they want" and still don't get fat. Some people just only want to eat an amount that doesn't get them fat. They aren't using any willpower or anything, they just don't want to eat more than a healthy amount for them. I am not one of those people (I am overweight when I eat as much as I want), but I am for other things. For example, I don't drink very often (only maybe 10 times a year) and don't get very drunk when I do. I just don't feel a desire to drink very often. It takes zero self discipline to not drink for an extended period of time because I don't have the desire for it. On the other hand, many alcoholics struggle mightily to avoid drinking. It takes all their willpower to not drink. Everyone is different in their ability to avoid different addictions.