The objection that so much SF is really stupid doesn't mean that it's a subset of fantasy. To my mind, it's exactly the idea that it's all the same which inevitably leads to the conclusion you can write any stupid thing in SF. Sure you can always write badly. But in SF, the style is supposed to be naturalistic, not supernaturalistic. Stupid, nonsense is bad SF style. Knowing what the alien ate in Alien would have been superior style. I was so shocked and horrified when John Hurt's chest burst open I didn't wonder too much how such an awful creature could grow. But if I had been quicker I would have found it harder to willingly suspend disbelief. If we're supposed to believe something fantastic is still somehow a natural phenomenon (potentially, for artistic purposes at least) then it is just good writing to help us out a little.
I suppose it's tempting for the fantasy apologists to focus on substantive issues. But again, I think style is really important in writing. The science in SF is always fiction, pretty much (barring near future thrillers, where---amazingly!---some readers promptly deny it's SF!) So no, being writer ignorance of real science and technology doesn't make it fantasy any more than a director who doesn't know Newton's Third Law of Motion is making a fantasy movie. SF based on outdated science doesn't suddenly turn into a fantasy. And if the only science a SF writer knows is homeopathy, the fact that's idiocy still doesn't make it fantasy either. Yes, it's only style. But in writing style is, well, some people say style is everything.
I suppose it's tempting for the fantasy apologists to focus on substantive issues. But again, I think style is really important in writing. The science in SF is always fiction, pretty much (barring near future thrillers, where---amazingly!---some readers promptly deny it's SF!) So no, being writer ignorance of real science and technology doesn't make it fantasy any more than a director who doesn't know Newton's Third Law of Motion is making a fantasy movie. SF based on outdated science doesn't suddenly turn into a fantasy. And if the only science a SF writer knows is homeopathy, the fact that's idiocy still doesn't make it fantasy either. Yes, it's only style. But in writing style is, well, some people say style is everything.