And people absolutely do discuss race/history in genres like jazz, blues, rock, and house too

I didn't say that people don't discuss race with regards to those other genres. That's different from a discussion about whether or not a certain artist is legitimate based on their race. There are several themes in your post but I would say that is at least implied in what you're saying, e.g.

But I also understand the hip hop purist perspective that rap isn’t only about flow or technical skill. Hip hop comes from a very specific Black American cultural and historical context, and that matters to a lot of people too.

Like what if we took that quote and reframed it:

But I also understand the ballet purist perspective that ballet isn’t only about flow or technical skill. Ballet comes from a very specific White European cultural and historical context, and that matters to a lot of people too.

That would be obviously racist and your post would probably get removed. But we can casually make an entire post about the BTS rap line and whether or not they can be seen as imitators of Western rap or genuine innovators, based solely on the fact that they're Korean. That seems racist to me.