Seems to me that they have juxtaposed many different metrics of "fitness" in this ranking.
I would never say that Phelps isn't fit. I know of few people who can swim the distances he can in the times he does.
So, it depends on what you value as "fitness."
"Look son, there comes a time in every man's life when he has to make a decision. Do you want to be big, powerful, jacked, yoked-up, have women everywhere want you and men fear you . . . or do you want to do crossfit?"