Dybbuks are not actually demons; they are wandering dead spirits which possess the living.

I think they do in fact make that distinction, but Goyer also wants to talk about stuff from "before time started", so...perhaps he's implying that either this thing isn't a dybbuk so much as a thing that's been called a dybbuk under other circumstances, or that while it was alive once it's been dead for so long now that it doesn't behave like it was ever human anymore. Or WHATeva!