Okay, Wright's claims that the PM doesn't say Scum=non-Terran are technically true, in the sense that those are not the exact keystrokes used. However, the PM does very heavily imply that Scum are always non-Terran, so the fact that Wright checked his PM and disagrees with this is really weird.
Wright wouldn't just come out and claim something like that if all he had was a Scum PM that doesn't say that. He's either town or he thinks he's town. It seems very clear to me that Wright has a PM that doesn't explicitly say that scum are never Terran. I think he's just trying to consider the possibility that the nuker might be a rogue Terran. The OP certainly leaves that possibility open, but the PM implies that this isn't possible. Wright's PM doesn't. It's fucking weird, but it doesn't mean he's mafia, though it could mean he's a third party.