Don't know what this has to do with anything, but....
I joined a swimming club almost a year ago. Long story short, my front crawl "flutter kick" was / is pitiful. After the glide from the wall, my vigorous efforts could offer no forward momentum; I just gently drifted sideways, into a lane rope. So I started trawling the 'net for tips and lessons on kicking technique.
Now, this might not be the be all and end all but, a runner's ankle is not suited to good propulsion with a swimming kick. An elite swimmer has very flexible ankles, which commonly enable their toes to point beyond "downwards" (their knees can also "hyperextend - bend the other way a bit). Very good for flipper-like propulsion, but pretty unstable and injury-prone, if you try to run around a bit.
I know even an elite swimmer doesn't get much propulsion from their leg kick, but at the very top of their sports, even that tiny amount might be enough to reinforce the belief that "black people can't swim as well as white people".