While not impossible, the text that remains is 240 pages (each page roughly 6.3 x 9 inches). Being as it seems to have some coherent themes across sections, it seems rather unlikely that a disturbed person could have written it on a whim.
Never underestimate outsider art, especially if crazy outsider art. Look at the works of Henry Darger and his book The Story of the Vivian Girls, in What is Known as the Realms of the Unreal, of the Glandeco-Angelinian War Storm, Caused by the Child Slave Rebellion which is over 15,000 typed pages. Along with that he has countless artworks associated with it, a 10,000 page handwritten sequel, and some other books including The Story of my Life which after 200 pages of talking about his life goes into almost 5000 more in a fiction story about a tornado he probably saw. Nobody even knew this otherwise unremarkable guy was doing this till he died and they found all this stuff in his apartment. It probably isn't too far that somebody could have done something similar but in their own secret code that only made sense to themselves.