Are the Edgar Rice Burroughs Martian Tales books science fiction, or fantasy?
Its written as a classic sword & sorcery fantasy story. But it takes place on Mars, and features aliens and flying machines. If Lord of the Rings had aliens and airplanes, but everything else more or less stayed the same, is it still high fantasy, or is it now science fiction?
Personally, I call them Science Fantasy, as they blend Fantasy and Science Fiction, but also don't belong in only one or the other. Fantasy Fiction wouldn't make sense since fantasy is always fiction already. Science Fiction often doesn't focus on science, but it usually features elements that are scientifically plausible, or have a science-back explanation as opposed to something purely fantasy like magic. So combining the two makes sense to me. I don't see why it cant be here.