I just looked up the IRC character limit (sources vary, but it's about the length of four Tweets) and I think it might be below the threshold at which superintelligence helps enough. (There must exist such a threshold; even the most convincing possible single character message isn't going to be very useful at convincing anyone of anything.) Especially if you add the requirement that the message be "a sentence" and don't let the AI pour out further sentences with inhuman speed.

I think if I lost this game (playing gatekeeper) it would be because I was too curious, on a meta level, to see what else my AI opponent's brain would generate, and therefore would let them talk too long. And I think I'd be more likely to give into this curiosity given a very good message and affordable stakes as opposed to a superhuman (four tweets long, one grammatical sentence!) message and colossal stakes. So I think I might have a better shot at this version playing against a superhuman AI than against you, although I wouldn't care to bet the farm on either and have wider error bars around the results against the superhuman AI.