You're getting downvoted because it's an oversimplification to the point of inaccuracy.

The variability within the groups far exceeds the variability between the groups. In other words an elite white swimmer and an elite black swimmer are much more physiologically similar than the white swimmer is to the random white guy or the black guy is to the random black guy. It's common for a fit white guy to be negatively buoyant.

Buoyancy is more and less a factor depending on the stroke. So in strokes were buoyancy is less of a factor, it would make sense that black athletes "over achieve" relative to the other strokes. That is not the case.

It's fundamentally a demographic, economic, and cultural thing. I have little doubt the build of an elite NBA small forward is has the phsyiological potential to be an elite swimmer if starting early. But they likely grew up with more access to a court (especially year around), likely had family/friends already playing, likely played for school to get into college, etc. Then take into account even a mid level NBA player will make seven figures a year with 8 figures in earnings. An elite swimmer is lucky to qualify for some world's and 1-2 Olympics, and make a fraction of that.