A lot of people who get called TERFs absolutely don't identify as feminists, let alone _radical_ feminists. It's just a meaningless term now
The word feminist is also a hard one to pin down. SOme people define it as just meaning believing in equality - but in a society where that is the consensus view (its not true of every society, but it is where I most frequently come across the word) that has little meaning.
In addition to the other replies, plenty of words are descriptive until they get used offensively. Another example is "retard".
I dont know, I've just seen it used in a seemingly derogatory way. I don't know what it means and I don't care. Nobody in my real life uses that term anyway, it's just weirdos on Twitter and such.