Yeah, Joss is a great writer (one of my favorites, tbh), but a bit of an idiot when it comes to these things. He suffers from what a lot of white male "progressive" writers suffer from, which is an insane characterization of every other male in an attempt to prove how much of an ally they are.
Of the comics I read (I only read 5 or 6), about 50% of them contain a gay black male in an otherwise all-white cast. Instead of just admitting that, as white men who grew up around white people, their imagination tends to skew white, they insert one background person to fill all diversity quota then spend the rest of their time attacking other white males for not being as enlightened as they are. Jon Scalzi is the same way.
I'm currently in school for writing and I'm, of course, the only black person in the program. These people spend all day on Twitter and Tumblr talking shit about racist, misogynistic conservatives, then send me their work to read and it's literally 100% rich, white people. I don't see a problem with rich, white people writing about rich, white people, but their blindness to their own hypocrisy just makes me shake my head.