Ah, ok, I was thinking strictly of the first definition, not of the second.it is a self-identification used by certain schools or traditions of second wave feminists but which can also be applied to people who use the same arguments and rhetoric as those second wave radical feminists.
Way back when I was first really starting to read about feminism, I was mostly reading contemporary Radfem stuff written by self-professed Radical Feminists (in hindsight, rather less critically than I should have been), and while some of them were transphobic in a similar way to Rowling, they seemed to be very different in other areas.
Though, just googled, and it turns out that Rowling was quoting Andrea Dworkin as recently as 6 days ago.