As far as I can tell the gibberish text eventually becomes readable. So the book exists and can be tried out, but with some annoyance. That still qualifies for Show HN. For books, people usually put a free chapter or two up, which is arguably less annoying, but there is room for different approaches.

The intention of the "try out" guideline is to make sure that the work actually exists and that people can play with it to get an idea of what's there. It's not intended to force Show HN projects to all be free.

https://news.ycombinator.com/showhn.html