Is GRR Martin’s ASOIF not every bit as much a work of Great Science Fiction as it is a work of High Fantasy?
(Though, perhaps given the numerous bloody, torturous, criminal, immoral, and amoral events of the story and books, perhaps Epic Fantasy is a far better term than High Fantasy. I should also say that I have read quite a bit of Martin’s science fiction so I do not make this observation in a vacuum.)
In any case look at the background, the events, and the milieu of the world itself. Even the very planet is apparently out of sync, ecologically and biologically. You have a world whose very orbit and rotation seems seasonally misaligned.
You have a past superculture (Valyria), apparently with a fairly highly developed technology, who were abruptly and almost instantly annihilated in what appears to be a self-induced immolation or act of self-destruction.
You have incredible acts of architecture, engineering, and materials control, such as with the Wall.
You have what is essentially a wholly alien race of creatures (the White Walkers) who can disappear into hibernation for untold aeons only to reappear in a mutated and far more dangerous form. You have other species of peculiar natures and seemingly bizarre backgrounds, such as the giants and the Children.
You have a very dangerous long-term degenerating disease which looks very much like some form of designed biological agent. Or yet another mutating agent.
You have a boy who cannot only “warg” himself backwards in time to gain critical information or historical events, he can actually influence people in the past. In other words you have visionary time travel with a built in ability to influence previous timelines.
And I could list many other such elements, including the dragons themselves, and their obviously native and possibly enhanced, not animalistic intelligence.
Now none of these things negate the obviously fantastical elements of the story (whichever you take as the source material for the real story and the truer events, the books or the Game of Thrones show) but they do point out that the frontier between fantasy and science fiction in this case is an extremely thin line of division.
Then again the exact same thing could be said of Tolkien’s work.
The frontier between science fiction and fantasy in the works of both men is an uncertain one indeed. At least when it comes to certain obvious elements.