While it’s hard to rule out genetic abnormalities, there is a huge difference between being fit and the kind of physical trauma associated with absolute peak human performance.

Plant based protean is often used for pure muscle building simply due to cost. With actual vegan examples being Barny du Plessis a bodybuilder, and Kendrick Yahcob Farris a weightlifter.

In terms of endurance Jack Lindquist a track cyclist shows that’s likely viable for the overwhelming majority of people. So while I think it likely takes more effort that’s in part due to market forces and economy of scale not inherent physical differences. If anything the higher amount of calories burned by top athletes often mean they need a larger quantity but lower percentage of protein in their diets.