I agree that a more fine-grained classification is more useful. However, the fact that all the greatest long-distance endurance runners are of Kalenjin descent

I agree that a more fine-grained classification is more useful. However, the fact that all the greatest long-distance endurance runners are of Kalenjin descent (Nilotic race) tells us next to nothing about any other trait. All the best sprinters are also Africans – but predominantly of Bantu ancestry, pretty diverse in this case. As are most NBA stars, and the great proportion of other athletes defined by a rather obvious subset of physical excellences linked to fast-twitch muscle fibers. At the same time, somehow no Bantu group excels in swimming. So using this method of yours, we derive that a great deal of a people belonging to a major race can have a similar ability profile in sports. Not too good for your argument. If you have to refer to non-central examples, this only shows that your actual case is weak.