The award is about the best book, and if that means a literary author writing sf rather than a sf author, then so be it. I don't have a problem with that. The Testament of Jessie Lamb
, for example, was published as mainstream and even made the MAN Booker longlist. I've also read it and it's very good.
But yes, the remainder of the list is very heartland science fiction, though whether that means they're good books I've no idea. I've read Embassytown
), and heard mixed reports on the rest.