As an any lifter, bodybuilder, or serious student of fitness and nutrition will tell you, increasing muscle mass substantially requires caloric excess. It is not impossible to put on some very modest amount of muscle mass while losing some amount of both fat and weight but it is very difficult. It requires keeping the calorie deficit mild, getting enough protein in, and timing that protein so that it is there when the body needs it. (Which revisits the frequent meal comment. Muscles can only absorb so much protein at a time, 15 to 20 grams inclusive of adequate amounts of essential amino acids. Getting that in in the window around resistance exercise, typically immediately after, and in multiple smaller meals later during recovery is widely believed to optimize muscle response to exercise.) Someone running a caloric deficit enough to lose the standard successful 2 to 3 pounds per week (and this doofus claimed to lose more) will, with a significant resistance exercise program and good timing of nutrition as above, possiby manage to not lose much much muscle mass, maybe even put some very modest amount on, but NOT anything like what this person (be he a fraud or merely delusional) is stating.
Again, the plan is not a bad one; the results claimed are just absurd. The principles are sound.
Think of it this way - the brain controls what and how much you eat, right? For most people there are two main inputs that factor in: satiety and hedonic value. Satiety is the metabolic system telling you that that is enough; high protein and high fiber foods tend to be high satiety. High satiety foods, all else equal, will result in less consumed. Hedonic value is pleasure/reward aspect of the food that tells us to eat more even though we are no longer hungry. Sweet, salty, fatty have higher hedonic value thus there is always room for dessert. The Big Food Industry has invested mightily in figuring out exactly what fires those reward centers as that drives more consumption and thus more sales. If those foods are lower satiety all the better. A limited selection of choices will be boring and not fire those centers so much. If all you were allowed to eat was a balanced bland people chow the odds are you would eat less than if you ate meals of bread and butter, appetizers, several items main course, dessert, and wine or other caloric drink. (The neurobiology of this is being worked out.)
If someone did force themselves onto a plan that was high satiety foods only and of such limited choice that the hedonic value was fairly low they would consume many fewer calories a day than those who eat the Standard American Diet (which consists of many low satiety hyperpalatable foods).
Thing is that such a nutrition plan is, by intent, boring, and who wants to be bored eating all the time. Hence his one day off and other people compromising and at least avoiding the hyperpalatable foods but allowing enough variety of high satiety foods that palatability is acceptable, a moderate level, just not the hyper level that most of us eat all the time.
Yes, even that can still be eaten to excess, as panche45 points out.
