It is objectively true in the history of the internet that 4:13 is the most common distribution for a 17 player game. 1:4:12 is also a thing. 5:12 is a very uphill game for town. I don't think its likely at all in this game (10% chance, MAYBE?). I expect Celever will agree with what Im saying here (and I think he does). So its unusual for him to say its "most likely" a 5 mafia game. Like yes it could be, but its unlikely given the context of not only the history of forum mafia, but also Smogon in particular.
I strongly assume this is either a 1:4:12 or 4:13 game. If Celever is scum, and is on a 4 person scum team, I dont think its too likely he would just make up that its "probably 5" and then backpedal to saying oh ya its really probably 4. Why would he do that as mafia? Granted, I dont normally like to indulge the "why would a player do X as mafia" line of reasoning because WIFOM is a thing.
Still, it seems far likelier that Celever's explanation that he said something out of hand and then later did the math is true than he said something (objectively) false as scum which he in particular should be expected to know given hes a forum moderator and game designer and reviewer.
That said, if this IS a 1:4:12 or there is a 4+1 (traitor) scenario, I think I would say Celever could still be an SK/Traitor and "slip". So if we end up flipping 4 scum and the game is still going I would give him a hard look for sure.