I mean it's not necessarily a dumb idea to think that someone with a socially unacceptable prejudice would seek to conceal it, only for it to slip out in unguarded moments.
I mention Corbyn because for quite a while I took the "moderate" view that he probably wasn't antisemitic, at least not consciously, but he had a massive tin ear on the issue which wasn't helped by being surrounded by sycophants who told him there was no problem. I didn't get any credit for taking the moderate position; actually I got scorn poured on me by assholes like James Meadway who would later reinvent themselves as honest brokers trying to solve Labour's antisemitism problem.
But then there was one incident after another that made the charitable interpretation harder to hang onto. And then there was that election debate where he was asked about Jeffrey Epstein, and bizarrely overpronounced Epstein's name to make it sound more foreign - like Ep-SHTOIN.
As David Baddiel commented, every Jew in the audience heard that.
There really wasn't an innocent explanation. And the next day I went on Facebook and found a bunch of friends tying themselves in knots to say they never knew Epstein was Jewish, so how on earth was Jeremy supposed to guess, and anyway Jeremy is such a lovely man who prides himself in pronouncing foreign names correctly that he was only giving Epstein's name its correct German pronunciation...
You get the picture. They couldn't believe their idol might be flawed, that there might be some truth to the accusations against him, so reality had to be rearranged to make him the innocent, wronged party facing yet more false accusations from Zionists like Baddiel.
So I don't rule out that these things can happen.
And I'm not emotionally bound to defend JKR as a flawless person. I'm not on X, but when I was she frequently said things I disagreed with. I appreciate her advocacy for women's spaces, and I enjoy her detective novels (though I wish she'd make them a bit shorter). I think she has many admirable qualities, and I'm sure a few flaws as well, because we all do.
What I don't buy is that her clear and consistent condemnation of antisemitism is performative and therefore lying, while her published work is littered with antisemitic dogwhistles revealing her true views.
I'm sorry, but I don't see an evidential basis for that. It's a theory that is popular in those parts of the Harry Potter fandom who start from the premise that she's a full spectrum istaphobe and then pore through the books looking for problematic elements to prove their. It was being pushed on YouTube years ago by the likes of Contrapoints and Lindsay Ellis. But those are not sensible people arguing in good faith.