'Despite being an SF Masterwork, Unquenchable Fire is very much a work of speculative fiction.
SF can stand for Science Fiction, Speculative Fiction, Science Fantasy and Structural Fabulation. Robert Heinlein and Harlan Ellison particularly argued (a long time before the likes of Margaret Atwood tried unsuccessfully to distinguish between Science Fiction and Speculative Fiction) that Science Fiction should more accurately be called Speculative Fiction. In other words, they are the SAME thing. This is why using 'SF' as a descriptor is more useful than either and why it is the professional's preference above the dreadful journo neoligism 'sci-fi', which sounds dismissive and is regarded as pejorative by writers, critics and serious SF fans.
In short, Speculative Fiction IS Science Fiction. All fiction speculates by default, so SF is extra-speculative. The elements in SF that make it SF are explained by the author of the individual work as scientific novums (the novelties that take the work out of the realm of realism) - this is sometimes done in a very subtle fashion. Novums that have a supernatural - or non-explained origin - are more broadly fantastic than those based on scientific speculation, so they are Fantasy.
Stephen E Andrews, author, '100 Must Read Science Fiction Novels', '100 Must Read Fantasy Novels'