Having something like another planet or alien biology is necessary but not sufficient to make something science fiction. There is some fuzzy amount of focus on science/technology, style, and tropes that are needed to make something more science fiction than fantasy or speculative fiction. The Left Hand of Darkness read more like fantasy to me, and I don't think it would have been any different at all if the setting was Middle Earth and the ambisexual species were Hobbits. I think I wouldn't have blinked if it had been described as "science fantasy" to me rather than "science fiction".
I'll bump up Chiang on my reading list, I have heard a lot of good things!
I'll bump up Chiang on my reading list, I have heard a lot of good things!