Re: Futuristic Fantasy?
I don't think membership of this genre by a work is purely a matter of whether putative scientific principles explain the events of the story or how things work in it. It has more to do with the style and the atmosphere of the writing plus a combination of the archaic (swords, aristocracy, hokey religion) with some advanced technology.
Another work occurs to me as possibly belonging to the sub-genre: The High Crusade by Poul Anderson. It relates the tale of a group of medieval knights & men-at-arms taking to the stars. And Anderson is commonly regarded as a hard SF writer (although he also wrote fantasies such as The Broken Sword).
Gene Wolfe's Book of the New Sun deserves a mention and M. John Harrison's Viriconium as well.