It comes down to intent but since everything is relativistically connected then there can be no distinction ever to hybrids and the like that teeter on the fence of the best of both worlds but there are touchstones and signposts and things and ideas that make something foremost science fiction and not fantasy. Serling was doing fantasy and dabbled in sci-fi when certain elements were present that indicated the direction, emphasis and thrust of the story towards being futuristic and involving physical technology or aliens, etc. that are the hallmark/landmarks of sci-fi. Lucas' mystical mythical force religion was secondary to the overwhelming amount of futuristic technology that was being presented, so there is a heirarchical order to the distinction of any genre - is that short for Gene R? - that distinguishes the various overlapping genres that may delve into one another and borrow heavily from science fiction but something has to weigh more and take presidence, if not to the author than at least to the audience.