The setting in the paper is about narrating a single combat between Ignatius J. Reilly and a pterodactyl. Ignatius J. Reilly is a literary character with some very idiosyncratic characteristics, that appears in a single book, where he of course didn't engage in single combats at all or interact with pterodactyls. He doesn't seem to have been the target of fanfiction either (which could be a problem if characters like, say, Harry Potter or Darth Vader were used instead), so the paper argues that it's very unlikely that a story like that had been ever written at all prior to this paper.
Well, we've been writing stories for thousands of years, so I'm a bit skeptical that the concept of "unlikely enough to exist" is a thing. More to the specific example, maybe there isn't a story about this specific character fighting a pterodactyl, but surely there are tons of stories of people fighting all kind of animals, and maybe there are some about someone fighting a pterodactyl too.
Sure, but the evaluation explicitly addresses (among other points) how well that specific character is characterized. If an LLM took a pre-existing story about (say) Superman fighting a pterodactyl, and changed Superman to Ignatius J. Reilly, it wouldn't get a high rating.
The setting in the paper is about narrating a single combat between Ignatius J. Reilly and a pterodactyl. Ignatius J. Reilly is a literary character with some very idiosyncratic characteristics, that appears in a single book, where he of course didn't engage in single combats at all or interact with pterodactyls. He doesn't seem to have been the target of fanfiction either (which could be a problem if characters like, say, Harry Potter or Darth Vader were used instead), so the paper argues that it's very unlikely that a story like that had been ever written at all prior to this paper.