In Homestuck? No. And I doubt it ever will be. In the fandom? Well, it hasn't until now because nobody had any information to contradict the beliefs they held before. But now Word of God says that their headcanon isn't universally correct, and for a variety of reasons (I'm willing to peg the major one as racism, to be honest) that's not acceptable to a lot of people. A lot of ridiculous, obnoxious people.
Eh... 4chan has had "arguments" about it, i.e. trollfests. There was a brief rash of depicting Rose as black (and not flatteringly) after the Grimdark thing, but other than that, eh.
In this case, I think the flareup on his tumblr was more about his original soap-box non-answer itself. People calling him a wimp for taking the "politically correct racial-rainbow" position instead of saying "yeah, they're (race) and that's not important". After reading them, I do think the second response was a bit necessary, because he got a little blunt with the idea of assumptions without really acknowledging that while it's perfectly acceptable to think of the characters as completely-non-racial, he did ground all of them in media stereotypes that say "white" (along with his acknowledgment that he once identified Bro as white, as a joke long before he put much thought into it).
Anyway, one great thing about this comic is that all it takes is a few days worth of antics and the subject changes. Even the fat-Vriska thing doesn't come up anymore.