There is a genetic component to it. His younger brother is exactly like him (he is only 17 and already is a machine in Crossfit).
Then there is his athletic background. He started training and competing for Crossfit last year. Prior to that he played okay for more than 10 years. Hockey is a "lactate sport" in that a shift on the ice tend to last between 40 and 60 seconds of intense work... and if the player is someone who is highly involved in the game his lactate system is taxed heavily.
In Canada, even with youngsters hockey is a year-round sport in that guys play during the winter (and early autumn/late spring) and have training boot camps/practices during the summer month as well as some summer leagues.
Add to that the off-ice conditioning work that revolves around the energy systems used in hockey.
I trained a lot of hockey players and most have a very high lactate threshold (their body delays the accumulation of lactic acid). So there is a training component to it. But it's something that you need to work hard for a long time on to improve a high level. And training for it is VERY unpleasant.
And I feel that the sport you did as a kid will play a big role.
Take Alex training partner Ben. Ben is an amazing athlete, one of the very best I've trained in my life. AMAZING strength relative to his body weight: at 170lbs he just snatched 265 x 2 with a pause at the knees and can clean 340lbs, bench press twice bodyweight, military press 225, etc. He is so explosive it's frightening. AND he has an amazing cardio capacity: he used to be on the junior national cross-country skiing team. He does the same training as Alex, is just as strong, has at least as much endurance if not more. But he has a very low lactate threshold... he builds up lactic acid very quickly.
See he has great endurance, but endurance and lactate threshold aren't the same thing. And in Cross-country skiing he didn't do as much of the later as hockey players.
So you can definitely improve your lactate threshold, but it will take a lot of effort doing very uncomfortable training to do that and someone who doesn't have it genetically will not be able to reach the same level as someone who does.