All this study really says, is that models are really good at compressing the text of Harry Potter. You can't get Harry Potter out of it without prompting it with the missing bits - sure, impressively few bits, but is that surprising, considering how many references and fair use excerpts (like discussion of the story in public forums) it's seen?

There's also the question of how many bits of originality there actually are in Harry Potter. If trained strictly on text up to the publishing of the first book, how well would it compress it?