Sci-fi/Horror = not a true horror movie.
I hate that sort of boxing in. Besides, Alien is much more Texas Chainsaw than Star Trek. Ridley calls it a horror film as do the writers and producers and cast.
Why isn’t a horror film set in space/the future a “true horror film”?
To me it seems like a Sci-Fi film with horror elements. Though I admit that could due to the threat being an alien rather than a crazed killer. I think we can all agree that Aliens doesn’t have a lick of horror in it.