The claim is that meat is "essential" to become big and strong. Which nutrients in meat are "essential" for this but not in a vegan's diet, so that they have to fill the gap with supplements to be competitive in sports?
For what one anecdote is worth, I'm friends with a former professional athlete who tried a vegan diet after after retiring - and years on he is still as freakishly strong and agile as ever. Many well-known pro athletes have gone vegan during their careers as well: Chris Paul, Venus Williams, Carl Lewis, among others.
This guy weighs 263 pounds, squats > 650, and eats vegan. https://www.openpowerlifting.org/u/ryanjstills