The enemy of my enemy is my enemy's enemy; nothing more, nothing less. JKR's platform belongs to nobody other than JKR, and how she chooses to use it is entirely her decision. In this instance, she is using it in a manner that is in line with basic biology and sanity, but remember her background and her past actions. She does not take this stance for the same reasons as those on the Right. She takes it because her feminist ideology demands she take it; her feminism is a through-line of her overall leftist - not liberal, leftist - political beliefs. She does not mind the censorship, the bullying, the harassment, or the egregious abuse of governmental power; she endorses all of those tactics when they are wielded in service of a cause she supports. What she takes issue with is the rubbish idea that humans can change their biological sex.
There is nothing wrong with applauding her when she's right about something, and antagonizing her when the tide is finally turning on troonism would be a mistake, but one should maintain realistic expectations based upon her past behavior. Rowling was never on the Right, nor will she ever be; flunking a Leftist purity test doesn't automatically reclassify her. She actively courted these people right up until the day they finally turned on her; she agrees with them on 99% of the issues. I'd love to believe that she's come around on censorship in recent times, but I'm not holding my breath. At least she's proven she can excoriate with the best of them, though, and it couldn't have happened to a nicer person.