A little reminder, we are talking generalities here, not about Greg Cox's writing, some of which I personally own, so no, this isn't about his badness.
But people, "horror" is not a big umbrella. Horror stories try to horrify the reader. Those are the only kinds of stories that fall under the horror "umbrella." To be sure, there are all kinds of horrors, including psychological and existential. That doesn't make it a different kind of story, just a different story. And a horror story that has some fantastic element that is presented with an effort at verisimilitude merely uses SF style. It still means to chill. One that has a magical threat uses a fantasy style has exactly the same aim. Other horror stories and movies use no fantastic elements at all. (Or at least not visibly, by accepted convention.) SF, fantasy, realist horror is all horror, not SF horror, not fantasy horror, not realist horror. No one has ever been confused enough to talk about realist horror. And that's why sticking a horror story into what began as an SF story was so jarring in Sunshine. Even if Danny Boyle thought he was just crashing genre barriers and none of it made a difference, he was just wrong. As for SF, that's written with an attempt at verisimilitude. That's what makes it an SF style. Fantasy is not written to seem "real." That's why it's really a kind of category error to mix them up.
As to the readers who don't distinguish between fantasy and SF, well, yes there have always been readers completely indifferent to style, valuing creativity and vicarious wish fulfillment much more highly. H. Rider Haggard and Edgar Rice Burroughs and E.E. "Doc" Smith made a living at it.
Can't speak to SUNSHINE since I've never seen it. Very possible there was a jarring change of tone that the movie didn't manage to pull off. As opposed to something like EVENT HORIZON (which, ahem, I edited the novelization of), which, as I recall,struck an ominous tone from the beginning. Hell, the spaceship itself looked like a spooky Gothic cathedral . . . ..
(Although I remember inviting a friend to a screening while neglecting to warn her that it was as much horror as SF. She was a bit taken aback, having apparently expected a straightforward space adventure or something. Oops.)
And, honestly, I think you can write fantasy or horror with as much "versimilitude" as SF if you're so inclined. Ultimately, Klingons are no more "real" than genies, so it's simply a matter of how "seriously" you approach the subject matter. Genre can be useful as a marketing tool, but why worry about "category errors" if it means you can't have robot vampires? (See "The Stainless Steel Leech" by Zelazny.) It's science fiction after all. It's an art not a science. To my mind, imagination and creativity trump any textbook definitions of what constitutes proper SF or whatever. Ultimately, we're all just telling campfire stories.
Granted, you pegged me with the Edgar Rice Burroughs thing. I devoured Burroughs as a kid, so that probably helped shape my personal tastes and views of science fiction..

(And don't worry. I'm not taking any of this personally. Never thought this was about my writing, just the pros and cons of mixing genres together, which is a fun topic to discuss.)
One can also argue that horror and SF have overlapped since Day One. See "Frankenstein," "The Island of Doctor Moreau," "The Invisible Man," "The Flowering of the Strange Orchid," even the blood-sucking octopoids in "The War of the Worlds"--which was probably the first "grown-up" SF novel I ever read, way back in third grade.
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