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.
Of course there are "big wins" in genetics. But they are the exception, not the rule. If you see someone smarter or faster than someone else, chances are genetics has next to nothing to do with it. Still we hear genetics mentioned in this context all the time.