I will play devil's advocate here and say Fisher's performance being praised in the final cut and test audiences wanting less Cyborg could both be true. It would not be the first time critics and general audiences disagreed on something, and it's not out of the question to believe your average theater goer wanted more Batman, Superman and Wonder Woman and less of the guy they might not have much (if any) familiarity with.
The thing is regardless of whether or not that's true, Whedon's response here was still dumb as fuck. Just apologize, promise to do better in the future, and then wait a few years before returning with a new project. Trying to take the fight directly to Fisher and the other actors by whining about them being rude and ungrateful and refusing to take responsibility has done nothing but make him look even worse than he already did.