If the model was trained over (say) a week during there was no full moon, how would the model know what this unexpected message means? It would probably just ignore it and continue playing as normal. Nethack is full of messages that can be safely ignored, so just another one would not be unusual.

I don't agree that the creators would be watching the game play either. Usually during such training phases you'd run as many copies of the game as the available hardware can manage. I wouldn't be surprised if they had at least hundreds of runs going in parallel and the researcher is definitely not going to be watching them all. If anything, they are going to bed and train the model overnight as much as possible.