I don't know about your statement "any runner that could make it at a shorter distance would". I don't know much about Jornet, but I seem to remember him hitting the ultra scene at a really young age, like winning that Mont Blanc 103 miles or whatever when he was like 18 or 19. So it seems unlikely that he entered the sport from a typical track running background, concluded that he wasn't going to be any good at traditional distances, and gradually moved up. It looks more like he entered ultramarathoning because it was just a challenge that appealed to him. Such people do exist. Not all ultrarunners are failed 5k/10k/marathoners.