Note that Pratchett does not claim that 'The Discworld novels can be enjoyed in any order', but merely that they can be read in any order. This is completely true. It is physically possible to read the Disworld series in any order. Indeed, it is possible to read Hogfather starting with the last word, then the second-to-last word, etc, until you reach the first. It would make almost as much sense that way as read front to back.