I had to look up what had happened, but not only did she tweet about JM when she meant someone else, she apparently deleted that tweet but posted another to ask what the difference was between JM and the other person, Penny someone, and calling Penny an 'irritant' and JM a 'social anthrax'.
So after her mistake, she pushed it further.
I think JM was right to take this to court. It wasn't just a mistake, there was a deliberate insult after making the mistake that implied JM supported something she doesn't, and if JM had ignored it then the mistake wouldn't have been corrected and people could still be thinking that she was responsible for the comments KM attributed to her.