Some people may think that the failure to make swimming progress was a testament to the current, but if you recall fully, it's actually just a callback to Kinnikuman being an awful swimmer. He was one of the worst swimmers back in the Choujin Olympics, and never even learned how to swim well enough to teach his son, Mantaro, how to swim, as shown in Kinnikuman Nisei.
So yeah, the whole "Kinniku Swimming Style" is itself just a dumb joke