I will say this, there's n0 impression I've gotten from Joss that would lead me to think he is racist. Or Johns. Now, that's not saying they couldn't have some implicit biases and therefore treat a young black actor or actress in a way they wouldn't otherwise a white actor without even realizing they are approaching them differently, but beyond something like that, I'm not so sure. I mean in the more overt kind of way that Fisher seems to be alluding to.
For Joss, he has had numerous black characters as main or important characters in his shows and movies and did not write them as one-dimensional (just a few off the top of my head are the slayer before Faith in Buffy, I forget her name at the moment; Gina Torres who was a prominent character in Firefly/Serenity and who was a main villain in Angel; the actor who played Shep/Sheppard in Firefly; SLJ as Fury, though of course Fury was already a part of the MCU, but Joss did more with him; Harry Lennox in a prominent role in Dollhouse; plus Joss has had quite a few other non-Caucasian characters in prominent roles on these shows).
For Johns, he's a big Cyborg fan and I think he's a John Steward fan as well (perhaps not over Hal, but c'mon, it's Hal freaking Jordan). He wr0te the new Rorschach as a black character and made him one of the hearts of the Doomsday Clock story.
Now, I'm not saying having non-white characters in your writing and in your movies and tv shows automatically means you can't be racist. I wouldn't go that far as there has been criticism leveled at Joss for being sexist despite how much strong women are the center of his works. I can see Joss and Johns being reputed for being jerks who may have taken advantage of their power, sure, but the racism comments from Fisher surprised me. For now, as with all of this, I will wait to see what comes of it. As I've said before, Fisher has always struck me as a thoughtful, deliberate man who isn't prone to letting his emotions lead him. But this is the first time I was surprised by what he shared.