> Arrival is arguably science fantasy since it's central conceit isn't very sciencey
Read the original short story: the actual premise is actually very science-y, and completely "true sci-fi." Don't want to spoil it here though.
> though everything _except_ that is very hard sci fi.
Yeah, personally I think sci-fi (rather than sci-fantasy) is allowed to break a single physical law and still be considered sci-fi, as long as it's done consistently and the consequences properly explored.
Personally I like James Nicoll's definition of hard science fiction: it's hard SF if the author gives enough details that the reader can be _certain_ it wouldn't work.
The Expanse is borderline since there's basically a magic substance (the proto molecule), though everything _except_ that is very hard sci fi.