It was implied dorm system is goona help find traitor but we dont know if it got any real results. I dont think it proofs anything
Nezu cleared all the students sometime before the war started iirc, and said that he'd continue to look for anything suspicious in the future when the league take action again. The war has happened now and UA has doubled down on security and the like, student traitor just doesn't seem likely, especially since there's no truly suspicious students (other than Aoyama, but he's just a red herring).
But, now is the perfect time for whoever the traitor is to take action, so we'll see what happens.
The thing is, the traitor hasn't acted since the training camp festival, before the dorm room system was set up.
True tbh, the traitor is so inactive that it feels like one doesn't actually exist. It's rather annoying and is an awfully written plot point. No matter who it ends up being, I don't think I'll care. At least it could make one of the 'boring' background characters interesting to a degree.