Just using some basic math and science here: 20 lb at 3500 cal/lb = 70,000 calories of fat he would have to burn in a month.
Assuming a month is 30 days, that is (70,000/30) = 2333 calorie per day deficeit.
With no exercise, a generous basal metabolic rate for a sedentary adult male is 2500 calories per day. Interesting that he only got 170 calories of energy per day from all that food.
However, if he’s doing no exercise, as stated, the advertised muscle gain is certainly magical.
So, yeah, it’s bullshit.
The unglamourous answer as others have said is counting calories. Nobody want to hear this though and they’ll make every kind of excuse, most of which break the laws of thermodynamics.
In my personal life, I can be hard to take, because I’m very blunt about these things. I speak as a 41 year old male who has heard all the excuses because he has made all the excuses. I was overweight from my late 20s to my mid to late 30s. It wasn’t until I saw a certified nutritionalist and learned calorie counting/portion control that I lost 50lb, going from a chunky 6’1" x 225lb to where I am now. It sucked, I was hungry and hated it, but I learned how to eat properly and have kept the weight of with minimal effort since then. Limiting calories within the confines of a balanced diet is the key.
My motto: If your diet as a name it’s a bad diet.
Side note: I saw this one literally yesterday: 2 twin brothers tried different diets - one cut out carbs, the other fats. The results were completely as I would have predicted.