The space opera equals science fiction mistake doesn't bother me so much; it's the ghosts and vampires equal science fiction mistake that's more troublesome.
On the other hand, I admit I've grown a bit weary of the constant protests that such-and-such "isn't really science fiction!"
Can we just stipulate that most articles or threads that talk about "The Best New Science Fiction Movies" or whatever actually mean sf, space opera, superheroes, fantasy, and horror? There's enough overlap that I don't really see the point in trying to keep the genres pure and separate.
Personally, I don't mind getting chocolate in my peanut butter. It's tastier that way . . . .