I would also presume that from a physical perspective he must be above average in ways that matter for this sport.
He seems to have a lot of lean muscle (http://media.npr.org/assets/img/2017/06/05/ap_17155772401614...)
gangly, adj., \ˈgaŋ-glē\ : long and thin [1]
It would be rude to call him a mutant, but that's pretty much what he is, for better or worse.
or the 80ies oldschool classic; wolfgang guellich in a german gameshow: https://youtu.be/rugoETlUAVI?t=22m47s
http://escalade9.wifeo.com/par-nationalite.php
i think what's so special about him are his mental abilities, not his physical.
He completed the Fitz Traverse with Tommy, who referred to him as the greatest climber of our generation(perhaps a bit of hyperbole, but still).
He is the current speed record holder with Hans Florine for The Nose.
Alex is the top ultramarathoner of rock climbing. Not only is his endurance of effort incredibly high, he is able to continually execute moves at an extremely high level with an almost non-existent error rate. He has admitted to feeling fear, and has mentioned that he free solo's stuff he feels very comfortable with. I'm positive his AMAZING physical abilities, particularly in the area of climbing endurance, are a contributing factor to his success in this area.
It may be true that his grade is not as high as 350 other peoples, however I would say that dismissing his "physical abilities" as not so special is perhaps not so accurate.