It's just such a weirdly silo-ed view I don't understand it. Like it requires ignoring the existence of all actual fanfic, stuff like the Magicians, professionally-published expansions of things out of copyright like Wicked, and heck it's written like most licensed fic doesn't exist either... like even if the Lewis estate enthusiastically ran with it, he'd be writing in the tradition of Ruth Plumly Thompson's Oz continuations.