Textbook definitions aren't that helpful when it comes to genre. There's a reason academics like to argue about these sorts of things all the time: they're not exactly clear.
' point that sf emerged from the fantasy genre is well taken. I'm not sure, however, that this origin in itself precludes sf from being considered as a genre in its own right.
In cinema, for example, what we know as the "musical" has its origins in the broader genre of comedy. At first it was "musical comedy" -- in other words, a subgenre of comedy. Today, I think most people would point to the "musical" as a distinct genre without a second thought.