Heh. Unfortunately, that idea runs into the same problem that the whole "joke/hoax" idea runs into... the book definitely dates from Dark Age Europe, when people were so busy struggling to just survive under the complete ban on the sharing of information that the Catholic Church imposed, and the material to make manuscripts was rare and mostly kept under lock and key by same. It's harder to imagine some monk playing a Dark Age version of D&D than it is to imagine them simply wasting the ink and parchament on a hoax.

My personal belief is that it was actually a hoax book, but one with a purpose: to convince someone, maybe some rival for advancement in the Church, that it contains critical information about the nature of witchcraft, and needed to be deciphered with haste (though the person who wrote it would know it could never be deciphered or even related to anything that actually exists). It wouldn't be the strangest bit of political maneuvering in the Dark Age, that's for sure. Once it served its purpose, that purpose was probably forgotten, but it was still remembered as being important enough to be preserved in some library or other until the first known record of its existence, in the letters of Georg Baresch, complaining about how it was "uselessly taking up space" in his library.