It literally cannot be as effective, because some people can eat a large amount of food in a short period of time.
What I suspect they mean is that the average person is more likely to stick to IF than they are to calorie counting. This feels intuitively obvious to me - it's a lot less work. Even on a bulking diet it can be really hard sometimes to figure out how many calories are in foods, small restaurants don't have a clue for example.