Robert Heinlein initiated hard fantasy.
John W Campbell edited it.
Anderson followed Heinlein's lead.
James Blish also wrote it.
Escape From Hell by Niven & Pournelle is in this category.
Has the Inferno been constructed by higher technology or effortlessly created ex nihilo by the will of a supernatural being?
If the former, sf.
If the latter, fantasy.
If the latter with logical consequences, then hard fantasy.
I think that Escape... is hard fantasy but it is always possible that a surprise ending will cause us to recategorize the novel.
Anderson's two Operation... novels are major contributions to this sub-genre.