I read some Roger Zelazny shorts the other day. I'm saying that the one about the chained dragon being sacrificed to a ferocious maiden was fantasy, but the ones about deep-sea fishing on the oceans of Venus and investigative anthropology among the natives of Mars were sf. But don't ask me why.
IIRC the Mars and Venus stories were deliberate homages to pulp SF. I wonder if SFF readers not raised on Asimov and Heinlein would classify them as fantasy, or if the planet names and rockets would still tend to keep them in the sf category.
Seth Dickinson's Exordia is science fiction that has literal magic in it (the sfnal element is how the protagonists work out how the sometimes horrific metaphysics operate); his Baru Cormorant novels are fantasies with extrapolations of weird scientific things instead of magic.
However, despite the possibility of the former thing existing, Brandon "hard magic systems" Sanderson writes fantasy. I will not be explaining my reasoning.
the mars and venus ones are fantastic, and the venus one in particular is very science fictional in most ways. i guess you could also do a fantasy story about building a massive industrial machine to catch a VERY BIG FISH but it would be a steampunk-y fantasy not a swords & dragons fantasy
just imagine that he handwaved in a faster-than-light drive to go to a distant star system & hunt the big fish if you want
There's a place for the as-you-know-Bob type of skiffy adventure ("so there we were, sweating in our suits (specially adapted for an atmospheric pressure 92 times that of Earth and an atmosphere consisting almost entirely of CO2), when I realised that our guide had double-crossed us...")
but I have a definite soft spot for Zelazny's approach, which is effectively to say "so there we were on a hostile alien planet - was it Venus? yeah, Venus, let's go with that" and then get on with doing something more interesting.
As I recall he said that he deliberately wanted to write a mars story and a Venus story before the actual science conclusively ruled out that fun.
(They were all a lot of fun, which some might say is the main thing.)