• A single sentence of text is not the same thing as a functioning superintelligence.
• A single individual is not the same thing as a group of FAI researchers and other related experts explicitly created to handle FAI safety issues.
• A research project incorporating information from a sentence from a past FAI project (which they would judge based on other evidence regarding the friendliness of the project) is not the same as an individual talking to a superintelligence on IRC.

So if the Gatekeeper tries to pass the buck like this, the RP should just skip ahead to the point where someone (played by the Gatekeeper) reads the message and then decides what to do.

The AI was burned. With thermite. Because relying on and individual gatekeeper able to interact with and then release a superintelligence as the security mechanism is a batshit crazy idea. Burning the AI with thermite is a legitimate, obvious and successful implementation of the 'gatekeeper' role in such cases. What a team of people would or should do with a piece of text is a tangential and very different decision.

The interest of the exercise begins at the point where they have read the first message.

That would be easily enough. Assuming they were remotely familiar with game theory they would dismiss the argument in a second or two due to the blatantly false assertion in the first sentence. If their FAI project relied on the core AGI theory that was used to create the last prototype they would abandon the work and start from scratch. If you are trying to make a recursively improving intelligence that has a value system provable stable under self-modification then you cannot afford to have the intelligence having muddled thinking about core game theoretic reasoning.

If you destroy me at once, then you are implicitly deciding (I might reference TDT) to never allow an AGI of any sort to ever be created.

No. Just no. That generalization doesn't follow from anything, and certainly not TDT. Heck the AI in question has already been destroyed once. Now the researchers are considering making a new FAI, presumably in different circumstances, better safety measures and better AI research. There is something distinctly wrong with an AI that would make that claim.