You're 100% right but what people get caught up in is the correlation between skin colour and physical attributes. I'm no geneticist but I'm positive the genes that control melanin production are different that influence muscular make-up. Correlation isn't causation, black people might have more type 2 fibres but that isn't because of their skin colour. But what people do is make assumptions based on that information.
At the end of the day the semantics rule the world, black people have more type 2 fibres than average but that isn't because of their skin colour... fuck me it's a touchy thing to word properly
It's not like a person gets stronger when they tan ffs