My take: It's not that Crossfitters are the fittest in the world and, on that basis, able to step right into a triathlon and do well. It's also not that they are fitter athletes than triathletes. The reason is that he's already an athlete who is accustomed to working hard, and in a structured way, towards some performance objective.
For starters, being the fittest in the world is impossible to prove, and that's assuming people could even agree on a single metric that indicates how fit you are compared to the next person. Second of all, Crossfit named their own competition fittest in the world. It's like when the NFL and NBA crown their champion the champion of the world, even though they are American sports leagues. American football doesn't even really exist in other countries...
The guy is just a beast and accustomed to putting in the hard work to achieve big things. He knows how to set goals, research coaches and training programs, stick to a plan, push himself hard, eat well, sleep well, and function as an athlete. Crossfit takes a lot of strength and endurance, but they do hard and heavy lifts. They are accustomed to the suffering that comes with training and competing. They can deal with it well.
And with that, I'm inclined to say someone like Kipchoge or any number of top, elite triathletes are the fittest people in the world.
Nourish - Sports Nutrition Made Easy