But, you ask, what about she-who-shall-not-be-named? She threatens defamation actions all the time, and it works.
And she does, and it does.
Because JKR is vindictive, wants to hurt people who anger her, has the money to do it, and doesn't really give a damn about her reputation.
And when she "wins," she gets an apology from one person and "JK Rowling is a holocaust denier" trending from the rest of the site.
To use a recent example
That's the difference that I think is easily missed here. JKR doesn't care about her reputation. She wants to shut up people and hurt them. Her threats don't do anything to keep her from trashing her own legacy and reputation; anything but.