Imagine having to play every single part of this game, explore every inch of territory, endure horrendous loading times, restart your console thousands of times, discovering bug after bug after bug, and still write a book that makes the game seem FUN.
Maybe he used a ghost-writer? You know, like in New Vegas when you use an explosive device and all the enemies turn into invisible ghosts?
Maybe he took advantage of all the frozen screens and lock-ups to proof-read his pages.
Perhaps he was able to foresee the future as some NPC would ask questions about a quest not even activated yet.
The guide is 448 pages. That's about 2 bugs per page.