I wasn’t, but I could have. Lewis didn’t have much imagination, really. And building a world requires more than just making up any story about some imagined world. Still, I don’t feel like quibbling with you over definitions.
So was The Hobbit.
I would disagree that Narnia was at all imaginative. It was of course imaginary, but that’s hardly the same thing. If you want to defend Lewis’s imagination, you will have to try something else.