Personally I think
- there is a difference between scifi and fantasy
- the term scifi often sucks, it should be something like future fiction? Most scifi I like isn't really about the science
And while the term speculative fiction is valid, it's an umbrella term, not an actual genre. Also I think most fiction is, at its heart, an answer to a "what if" question, so maybe the term is too broad.
Your example is tricky.
There's one novel of Iain M Banks' Culure series that is like that. Without context you'd think it's typical fantasy, all medieval-like, except for one or two scenes that are either weird magic or a knife missile/drone. Of course we know it's the latter.
So as long as it has even 1% of "scifi", even by association, I'd say it's OK.
All that said I much prefer to read good books, no matter the genre.
edit: !fantasy@lemmy.ml or make a new one, it's easy!