Ok, scoosh was just pointing out the content of the programme in your quote.
Cambridge dictionary online gives the following definition of the word diet:
when someone eats less food, or only particular types of food, because they want to become thinner or for medical reasons
I'm going on a diet next week and hope to lose two kilos before Christmas.
a crash/strict/calorie-controlled diet
The doctor put me on a low-salt diet to reduce my blood pressure.
I only used an online dictionary as I couldn't be bothered to get off my turbo trainer to look it up in the OED.

If a diet is when someone eats less food it implies that when a diet stops you eat more and the weight returns. Calories in = caloies out and any extra is stored as fat. Although that is a little irrelevant in my instance because as you say weight loss is a pleasant side effect of this diet rather than the be all and end all.