All fiction is fantasy. Fantasy is the root of all fiction. As soon as one starts to change reality, by lying about what someone said or thinking about what you could have said, that's fantasy.
All labels are either useful fuzzy groupings or non-useful groupings. "High fantasy" is a label that gets you works closer in some conceptual space to The Lord of the Rings than to The Manchurian Candidate. "Isekai" is a very definite label that applies to The Lion, The Witch And The Wardrobe and does not apply to Space Quest, but it applies more to Space Quest than to most episodes of Star Trek (except The City On The Edge of Forever, which is definitely isekai, and some similar episodes.)
"Science fiction" is a marketing promise that if there are elves, there will be an evolutionary heritage for them and a reason that you don't notice them walking to your local pub (no guarantees about the walk back.) Spock is a kind of half-space-elf. The space elves split into two groups a long time ago -- the ones who subscribed to a rationalist approach, and the ones who had a religion based on passion.
It is, but the author didn't base their argument on that entirely. More so, Star Trek has never cared about science except as set-dressing for drama. People say Star Trek's warp drives are based on the Alcubierre drive but they really aren't - they don't behave the way an Alcubierre drive should. Along with all other Trek technology it's basically magic, nothing is bound by realistic physics. You have immortal, godlike beings like the Q, you have people coming back from the dead, psychic phenomena, time travel whose rules change with each episode. In the last episode of SNW, you had a weird space anomaly that made people sing like a musical.
I mean, you could put that in a D&D campaign and call it magic and it would work.
Really, all science fiction is a subset of fantasy. "science fantasy" is just science fiction that isn't actually about the science, which is the bulk of the genre.