The athletes you peak of, most of them have gone through a tremendously lengthy, life long buildup of exercise tolerance. They didn't go from high school senior to engaging in 20+ hours of training abruptly; they built up a base since they were kids.
And they do a variety of training, not just lifting, so they're not stressing the same energy or nervous system pathways over and over.
It's preference and reaction to training. Dorian didn't do a large amount of volume, but he trained very hard. Contrast this to training longer and less hard. You can train hard and short or less hard and long, but you can't do both.
I'm a big fan of Dorian, but I grew a little less fond of his style of training as time went on. I think he might not have been so damn busted and injured had he at least did some higher volume training here and there.