Had anyone said this I would surely be agreeing with you but I think it was just one ambiguous sentence.
the initial one was ambiguous to be sure, and it didn't go quite that far but following ones did say just that: to decry homophobia in hip hop was nothing but high-and-mighty posturing. It isn't, for one thing, and for another it isn't pointless to complain about it, unless, as mentioned before, you are a defeatist who doesn't think culture, this or any other one, can ever change!
As for the point you wanted expansion on, I think it's simpler than you are thinking. An active response to an artwork can merely be a comment about it. It's better than deciding to not listen to it, in the same way that writing an article about how fast food is bad for people can have greater effect that merely making the personal choice not to eat at mcdonald's ever. Ideally you'd do both.
You are indeed right to say there's a lot of gray area here and the idea of how music lovers should position themselves in regards to the politics of the artists they listen to is a difficult issue. And I'd like to talk about it a bunch more because it is really interesting! I guess the only real salient point I am trying to make here is simply that the sheer complexity of our pluralistic, layered confusing culture does not absolve anyone from criticism, even if the critic cannot guarantee full understanding of the context of their work.
Nobody can guarantee that. Furthermore, it is not a pointless excercise to attempt to criticise elements of art and/or culture you disagree with, especially considering how effective that very activity has been in the last sixty or so years.