"Hard Fantasy"

The premise of Robert Heinlein's "Magic, Inc." is that magic works and is practised like a set of technologies. Magical practice is based on the reality of supernatural entities and forces, not on any new theory, discovery or application of the natural sciences. Thus, "Magic, Inc." is fantasy, not science fiction (sf).

We might call it "hard fantasy" to indicate that the implications of the premise are deduced as rigorously as are the consequences of any new technology in hard sf.

Two other "hard fantasies":

in The Anubis Gates by Tim Powers, there is time travel to historical periods with circular causality as in an sf novel but here the time travel is one of several applications of magic;

in Black Easter/The Day After Judgement by James Blish, demons are real.

Blish wrote mostly hard sf. It is possible, when reading his fantasies, to forget that they are a different genre from his sf. Indeed, some of his characters find it hard to believe that their high technology coexists with demons. In fact, Black...Judgement is the second volume of a trilogy about the conflict between secularism and supernaturalism. Volumes I and III remain ambiguous but it is a premise of Volume II that demons exist and are neither technological nor extraterrestrial but supernatural.