They didn’t argue that they owned the name “WallStreetBets”. They argued that having the founder of /r/WallStreetBets associated with an event named “WallStreetBets Live” falsely implies that Reddit is associated with the event. It was a weak argument, but they won anyway.
Indirectly, Reddit did assert ownership of the community. If you build a community on Reddit that community cannot leave Reddit and take its own name with it.