IIFYM is probably 95% of a diet. The other 5% can be nutrient timing, meal frequency, etc.
Why IIFYM work is because it limits your food choices (to an extent). What I meant by that is that the fat is usually the limiting factor. I have to select my food properly to account for that limit. So instead of going out there and eating chips and pop tarts (limited fat intake), I eat fruits, bread, pasta, and rice as my main source of carbs (fortunately I like them). Can someone go out there and eat candies for their carb intake? Sure. But sooner or later it might not be enough to fill them up as they hit their daily limit and will be forced to select food that provides more satiety.
Pretty sure IFBB pros follow IIFYM. It’s a part of every “diet”. I really dislike using the word…