Her training has nothing to do with the arm. She is a trained fighter but she acts nothing like it in the book. Because it's retconned.
Nico was learning different languages before A-Force was formed. She wasn't doing it for the team, she was doing it for herself (issue 1). But that's also retconned.
A writer ignoring continuity isn't exclusive to Runaways. It's just blatantly obvious to me because Nico is one of my favorite characters and I've been following her now retconned journey for a long time and I know her appearances in those books by heart. Rowell is distancing the book from those runs and I realized that bothers me. More now with this recent arc. It's still a great book so I can totally see why it doesn't bother most everyone else