I think she's wrong. A good children's book (or film) can be enjoyed by adults too. I didn't read the Chronicles of Narnia until I was in my 30s, but I loved them. As CS Lewis himself once wrote, "a children's book that can only be enjoyed by children is not a good children's book".
I'm not sure about extreme fandom, though: I think nerdy people like that are harmless, and generally likeable in their nerdy way (though I wouldn't want to spend too long with them, for fear of being bored to death), but that kind of single-minded obsession is not altogether healthy.