If they are the fittest, how come he can actually do it? If he can "win", that is fit enough for nature, moral questions aside.
If we had a whole system saying that people with 11 toes always run faster, and a person with 10 toes wins once, that system was based on false premises, taken as objective truth without proof.
Now, if we have a system that claims people with 11 toes usually/on average run faster, why not allow the diversity of runners with 10 toes?