The reason for the tweet was the wording “menstruating people”, used by the author of the article. Joanne Rowling found these words inappropriate and quoted them with a mockery, hinting that things should be called by their names:
Rowling noted that “mopping up the gender” deprives many people of the opportunity to discuss issues, and the desire, to tell the truth, does not hate at all. The reaction to the ambiguous joke and controversial position was not long in coming. Joanne Rowling was accused of transphobia not only by regular Twitter users but also by other celebrities. So, the writer pissed off actress Sarah Paulson, who suggested she shut her mouth.
By the way, this is not the first time that Joanne Rowling gets into a media scandal of a similar theme. So, in 2019, the writer publicly supported Maya Forester, who criticized transgenderism and lost her job because of this.