I really liked the bizarre setting they did, Fantasy London becomes A Medieval Sci fi city in the south african desert? Fantastic. Vimes running around dressed like a pirate with smoky eyeshadow? Great! I think though the whole story and writing was a mistake. Like even if you divorce it entirely from Discworld (which they should have done) its just not a very well written story.
This is the type of local history you can drive yourself insane trying to decipher. Local ghost stories are great for it, the best one I ever found was a theory paper about how the possibility that the British Army Intelligence purposely created occult panic during The Troubles in nationalist areas. Trying to discover the truth behind well known Troubles related stuff is impossible (sometimes literally) at times, nevermind something that, if it happened, would have never been admitted officially.