That's a fair point. Another athlete I do follow who is ripped[1], has been reportedly Vegan since he was 2. Generally speaking when I think of building muscle in the gym, I think of protein powder and supplements as opposed to eating ever larger quantities of meat; I don't think that's controversial? AFAIK the only thing you need if you are vegan is a Vitamin supplement.
I'm not a Vegan promoter or anything -- I'm not even sure what a Vegan eats for most of their meals (but would love to learn). But having some background in nutrition I'm skeptical there's any shortcoming to being Vegetarian, Vegan, etc, that a very simple supplement (i.e. like one regular vitamin) wouldn't overcome.
Most protein powder is whey-based (not vegan). Whey protein is generally inferior to muscle, connective tissue, etc. but it's typically a lot cheaper. It's better than plant-based alternatives though. Plant protein efficiency is terrible compared to animal protein efficiency, so any moderately muscular vegan is likely eating prodigious quantities of mediocre protein sources like peanut butter.
> AFAIK the only thing you need if you are vegan is a Vitamin supplement.
This is what some vegans will tell you, but you only need to look at the ones saying this to know it's not true. I know some vegans who could be mistaken for non-vegans from a fitness standpoint, and they put quite a lot of work into maintaining their diet. More than just taking a few vitamin pills.
OTOH, you can achieve better results by just eating a lot of steak...
BTW, famous vegan athletes almost never start vegan, and tend to quickly retire due to increased injury rates and slowed recovery.