Sure if you decide to defame Rowling she's entitled to instruct her lawyers but having a pop at Rowling wasn't going to get Labour Party members, including front bench MP's, expelled from the Party the same way having a pop at Corbyn was. And people were mostly interested in what the Labour Party had to say about anti-semitism inside the Labour Party and how it affected the factional balance inside the Party rather than the views of AN Other Internet User.