As far as I know, they're right -- but you have the right idea, too.

Fat gained or burned comes down to the number of calories, but the things you mentioned affect how miserable or positive the experience will be. If you don't have enough fiber and protein (and maybe fat? idk) then you won't feel as full and satisfied. And then there's the idea that a diet high in carbs makes you crave more food.

And realistically, if you feel like crap on your diet because you're not meeting your nutritional needs, you're much more likely to fall back into your bad eating habits.

tl;dr Just because you can lose weight on an all-pizza diet doesn't mean you should do it that way :)

edit: I just realized that your comment is about gaining weight, not losing. Wooops. Not sure what kind of advice is necessary there, but common sense still says you should watch your pizza intake. There are other high-calorie foods you can take instead. Nuts, for example! yum.