Not true.
There are many athletes performing at the highest level of sports (including powerlifting) that are vegan. I held this incorrect belief too until I watched that documentary produced by Novak Djokovic, Lewis Hamilton and the frekkin Terminator himself:
I had no idea there were so many athletes that were vegan, even in sports like the 100m sprint, which are pure fast-twitch muscle sports. The ultimate test of any diet is athletic performance. That is the only definitive empirical proof that the diet can work at the highest level and vegan diets have passed that test.
I've been trying out a vegan diet for about 6 months for environmental reasons (not every day, like 5 days a week), and my fitness level has increased. On the days I eat meat I feel more sluggish during workouts, versus my vegan days. But even beyond fitness, reducing meat intake is simply better for the environment. You can't have 7 billion people eating meat every day and have a sustainable ecosystem.