I feel like you skimmed what I wrote and missed some very key words.
I said the rest of the world ISN'T adapting and that it's cultural and that running is becoming more popular so there are more people doing it likely creating competition against the Kenyans. The point about Kenyans moving into metro areas is a very valid point, although the reading I just found suggests it's runners of a specific tribe rather than Kenya as a whole. So I suppose any statistical analysis would need to be done with the Kalenjin vs Everyone else.
In regards to genetics, your ability to become overweight is something you can be genetically predisposed to. Therefore it works the opposite way. However it's more likely their diet and culture influencing that. It also wouldn't be 1 trait, it would be a combination of prevalent dominant and recessive traits which in a region even as great as 5million people, can absolutely cause a "type" of person. Statistically there are some regions with very short people, there are some regions with very tall people, there are some regions with tons of red heads, there are some regions where the majority has 6 fingers on each hand. So if having thin ankles, thin calves, and being tall and thin are things that are genetic traits that make you more likely to be a good runner, is it not possible that within the Kalenjin tribe there is a higher incident of these traits being present than in other regions?
So a genetic predisposition to have a greater number of physical traits good for running + environment/culture = great runners.
Then there is Lactic acid... I'd be interested to read a study on the Kalenjin and how their bodies process it since most of the research is relatively new.
In regards to the lack of information on actual body statistics rather than weight and height... I suggest you look into Race and Anthropology the work of Franz Boas, and why studies of this sort are often avoided due to racial inferiority or superiority complexes.
