Yeah, a creative person can call their creation whatever they like, that does not mean it does not fit nicely in an existing genre.
He can and he did…That being said I would add a little more emphasis to his decision to call his creation that then some anonymous guy on the internet defines it. He wasn’t a rookie when he made SW and had been immersed in movie making and its history for some time. IOW he certainly knew what he wanted to call it.
Not some guy on the internet. We are talking an entire slice of the publishing industry (publishers, writers, reviewers, etc.). People can makeup whatever name they like for something they create and then it gets pigeonholed into whatever genre it fits in. Star Wars gets filed under Science Fiction/Space Opera.