If these are people who have grown up with a social community then it's not so odd. It's about being part of that community and relating to each other by internal language. It becomes not about Harry Potter but about real world relationships which are closed to outsiders. It's about a sense of belonging. Which I think, to a point, is fine really given we live in an era where local communities have broken down. As long as it isn't to the total exclusion to the rest of the world and it's something you can put aside when appropriate to do so.
It is no different to any other fandom really. Sci-fi and fantasy are full of them. And many adults indulge in it and you would never know as the rest of the time they do function as fully grown adults with professional jobs (going to lots of conventions and buying all the crap is not a cheap past time so they often need half decent jobs to do it!)
I think it shows an incredible narrow mindedness on the part of Miriam. But then she is like this on a number of issues. If she doesn't agree / like she decides it rather than trying to understand it. She's really nasty with her prejudices too.
I don't like her manner at all.