Back in the old days (60s and 70s), those of us who were into science fiction never called it "sci fi". We referred to it as "SF". Anyone who called it "sci fi" was looked down on as relatively uninformed about the genre. I don't know whether that is still the case among true SF mavens, but the more common usage is now used everywhere.
What makes The Expanse interesting, despite its glaring anti-science special effects (booming noises in vacuum), is that it is a good effort to portray a "hard science fiction" yarn in space opera form. Space operas are essentially dramas modeled primarily after popular literatures such as westerns, mysteries, and adventures. However, the venue here is some projection of a future that attempts to extrapolate from a perception modern technology or science. Soft science fiction is basically a fantasy story with scientific mumbo jumbo instead of magic or religion (e.g. Lord of the Rings). Most of what we see in the arts and media these days, to the extent that it could be called "science fiction", is of the soft variety. It makes little effort in the direction of realism.