They didn't say it would cause muscle gain, they said prevents muscle loss. Those are two very different claims

Protein has 4 calories per gram

Protein is only used as fuel in extremis, it's a dirty fuel and the body will poop what it doesn't need for repair if there's enough other calories coming in. Also only some amino acids are biochemically amenable to conversion to energy, whereas the 4cal/g figure is from burning it all in air. Protein does not turn into cake, you can however argue the excess is wasted, but how much is excess for a given individual is a hard question.

You're quite right about needing stimulus to grow, but the statement was about maintenance.

Perhaps, but nobody in this thread argued it built muscle, only helps retain it if you're sedentary.

Plus excess protein is bad for you. Your body just has to work harder to filter anything you don't use out.

Yeah the current marketing obsession with protein just screams fad diet. Remember "super foods" where you try to throw certain nutrient dense ingredients into all of your food? Whatever happened to that?

A cousin of mine who games all day started chugging protein for 3 years from 15-18. Dude became really bulky, like big, but not really muscular. Sometimes I genuinely can't tell if he's fat or 50.

I think a more correct term would be mass-growth, not muscle growth.

I got four hours of exercise today sitting on my arse

The didn’t say that. They say it encourages retention as opposed to muscle loss. It’s true. Get off your weird soapbox, your fact while true wasn’t relevant here.