I do understand what a SLAPP is, but it’s quite clear you don’t - or that you’re pretending not to, it’s hard to say.
JKR is constantly subjected to unpleasant and offensive criticism on social media - my facebook feed is filled with endless crappy takes from so-called influencers about what a terrible person she is. She is also, as we know, on the receiving end of vast numbers of death and rape threats.
She ignores most of this. If she wanted to, she could easily silence people by setting her lawyers on them - she is, after all, vastly more wealthy than most people. To her credit, she doesn’t. If it amuses her, she’ll sometimes take the piss out of people like India Willoughby or other idiots.
But if someone makes a statement that is obviously defamatory, and that person has some public influence (eg they’re a journalist), then she puts her lawyers on the case and extracts a grovelling apology.
Calling someone a Holocaust denier is extremely defamatory. Do you really need me to explain why?