They're not saying he did it because he's a vegan. They're saying that being a vegan didn't keep him from doing it. However, being a vegan obviously affected his mental capacities, because he
ran for 135 miles in Death Valley. If that's not crazy, what is?
Nonsuch, you laugh, but if you google "Paul Anderson Beef Juice" I bet you can find a detailed description of perhaps the strongest man in history drinking blood, fat, etc. from beef blended together. He thought it made a huge difference in his development.
Even if it worked, that's more dedication than I have.