I think the answer is much simpler than you think it is. I don’t think he was writing the book for any larger reason beyond needing a cover and career on earth.
If he wrote about all the adventures where he decimated planets within a day, those would get old fast and be relatively short reads. “I got there, killed everything and left.”
I don’t think the character had any reason to believe the books, or their contents would ever be needed for anything beyond entertainment.
At the point in which they were written, and the nature of the books being hard battles that required tact and training, their usefulness increased exponentially.
That’s why he doesn’t talk about why he wrote them beyond his career as a writer. He was just trying to write a story that was entertaining using the few examples he had of tough adventures.
Battle Beast will fight these people until his dying breath and that will give me plenty of time to run away.
Battle Beast because he fuckin rocks.