Do we know what it was they read at the end? Was it already explained somewhere? The statement by the mc 'this book is a work of fiction'(page 4) was just a hope by the mc it would be so, so it's not that. Do any novel readers know what was written?
Also, it's not the demon lord stuff, that would have been the first pages of the book they found that out, and they did indeed show their reactions earlier...