a good mind helps but ya need a strong cardiovascular system too... he’s built for it.
The Sherpa constitution goes well-beyond physical conditioning (it's not like the go running or jogging or something... they don't). It's DNA, genetic.
There are maybe a handful of Westerners who even come close to this, and have successfully climbed Everest without oxygen. Ed Veisturs and Reinhold Messner come to mind. Butu even they are outdone by your normal Sherpa high-altitude guide.