Look, I didn't say it had never meant the same thing, obviously; I'm not that clueless, but in my lifetime--which is admittedly not that long--I've never heard/seen the word "curly" used as a substitute for "nappy", "kinky", or any other clearly-derogatory descriptor. "Prior to the 1920's" isn't that long ago but it was a long time ago in the world of slang.

I'd also like to point out that I've seen the same, or nearly the same, songs with both words, which makes me suspect that they started out with an uglier pejorative and that "curly" was substituted later to make the more suitable for public performance, since it was less pointedly racist (at least, after the 1920's), in much the same way lyrics that were originally performed in heavily stereotyped black dialect eventually were converted to more standard language.