If we have the top marathon runner from Kenya, and the top marathon runner from, say, Russia - then chances are that genetics have everything to do with it.

At top of sports, every single one of them has all the "nurture" stuff close to maximum - not identical, not maximum, but with no huge differences in the training and preparation. If you see "someone faster than someone else", then if those 'someones' are a gold medalist vs an amateur - sure, genetics have little to do with it; but if those 'someones' are a gold medalist and 4th place... then genetics (or possibly doping) are everything; the 4th place runner did everything else pretty much the same as the top one.