I don't think it's quite that simple, but I do think that calorie deficit is the central factor for weight loss in most diets. I do think there is some role for timing and body state in the efficiency with which someone absorbs calories and how they metabolize them that can produce an effective calorie deficit between calories invested and calories that are usable and I think some diets capitalize on that. There may be some role of body-integrated calorie catabolism during ketosis states, but I'm less sure of the legitimacy there and haven't read enough to really know.