This is a perfectly sensible response to a silly and provocative question.

If this kind of banter counts as "transphobia" these days, I'm not sure the word means anything anymore. It seems you're delegitimizing and trivializing a genuinely serious issue.

Authors always have to reckon with the reality that not everyone will like their work, and not everyone will buy their work. You simply have to get used to it. The only way this would be a telling rhetorical blow is if JKR's sales were massively, massively down. She's letting the tweeter know that no, they're not, not enough to be financially troubling.

It really doesn't help anyone to label as "transphobia" an ineffective jibe against an author. Can't we save it for the genuinely problematic stuff?