Yeah, winning is trivial - you just don't open the damn box. It can't get more trivial than that.
I don't think you or Sly quite understand what the game is. The game is not "the Gatekeeper chooses whether to open the box, loses if he does, and wins if he does not." That game would indeed be trivial to win. The actual game is "the Gatekeeper and the AI will roleplay the interaction to the best of their ability, as if it were an actual interaction of a real Gatekeeper with a real untrusted AI. The Gatekeeper (player) opens the box if and only if the Gatekeeper (as roleplayed by the player imagining themselves in the role, not a fictional character) would open the box."
As the Gatekeeper player, to blindly keep the box closed and ignore the conversation would be like "winning" a game of chess by grabbing the opponent's king off the board. To lose by saying "hey, it's just a bit of fun, it doesn't mean anything" would be like losing a game of chess by moving your pieces randomly without caring. There's nothing to stop you doing either of those things; you just aren't playing chess any more. And there's nothing to stop you not playing chess. But the game of chess remains.
The game is not "the Gatekeeper chooses whether to open the box, loses if he does, and wins if he does not."
Actually the game is exactly this, anything the AI party says is just a distraction.
AI Box Experiment Update #3
Tuxedage (AI) vs Alexei (GK) - Gatekeeper Victory
Tuxedage (AI) vs Anonymous (GK) - Gatekeeper Victory
I have won a second game of AI box against a gatekeeper who wished to remain Anonymous.
This puts my AI Box Experiment record at 3 wins and 3 losses.