I think there is a distinction between a ghost story and a story with ghosts in - in the former case I'd expect the ghost to be pretty damn central to the proceedings. The Moonstone is a detective story; Emma, IMO, has elements of the detective story rather than being one. But all the best novels cross genres anyway, and the best writers are not silly enough to be ashamed of genre elements if the book needs them. Lerner's idea strikes me as plain bloody silly - "if it's produced by someone I consider to be with the SF community it's SF"?

can a poem of fourteen lines not be a sonnet?

Yes. But it can't avoid engaging with the sonnet, because any reader will at once think "sonnet", so if it isn't one, it needs to not be one and yet have 14 lines for a reason (I know what I mean, even if it doesn'r appear to make much sense...) I'd call it a deliberate non-sonnet.