All that means is that the invention of Hogwarts is sufficiently recent that the identity of its inventor is still known, that the original work of fiction is still available, and that nobody has yet published derivative writings which make a claim of literal truth.
Take scientology as an example, then. We know it was generated as a work of fiction for personal enrichment of its creator. We know who its creator is. We know its claims to be expressed very clearly. We also know that there is a large body of subsequent commentary which treats its subject matter as entirely factual. Are you agnostic about the existence of body thetans, and is it incumbent on everybody in the population to have a stated position on their existence?
Dave
The ridiculousness of a particular fictional religion (Scientology) has a much to do with the possible existence of god(s) as the ridiculousness of a particular fictional alien life (Vogons) has to do with the possible existence of alien life. Which is to say, nothing.