Okay, first, don't talk about what you don't understand.
If their employee has been completely silent and they're suing her instead of the people who were actually talking, then good luck with that case. Hence "named the wrong defendant", you name the person who is ACTUALLY causing economic damages, not the one being silent. Second, what are they suing her for? You actually do have to have a cause of action to bring a case. Breach of contract? For what, she hasn't done anything she hadn't been doing and they hadn't been tolerating before, and that arguably invalidates their contract claims because of prior tolerance. She hasn't done anything else, so good luck with finding a cause of action. I used "slander" in a casual, not legal context, she doesn't have a cause for slander (yet, but give them time because they're getting there the more they go in on this).
If I were their legal counsel I'd tell the company to shut the hell up (and I'm sure that's what their lawyers are currently saying, hence few statements yet about the situation). Lol, as part of a law firm we're always telling clients to shut the hell up. They don't always listen, but we try.
They surely filled a lawsuit for defamation which could backfire with Hyunjoo not saying anything publicly. Still if she helped her brother with the statements and they can't prove the bullying they are in the losing side.
We don't even know what is the lawsuit (they even only state legal actions) so i don't think we need to dig that deep.
This is a link to the last DSP statement with the legal action part : akplink
They claim Hyunjoo and her mother were working behind the scene but this is probably not defamation.