> saying "just" dismisses its influence on the genre as a whole.

Well, it was meant to be parsed as:

Star Trek is speculative fiction and space opera.

Star Wars is just space opera.

Some space opera is also speculative fiction, but I wouldn't say it is a subset. I wouldn't call some space opera stories speculative fiction at all.

They're all classified as science fiction.

(Yes, yes - there is no consensus on these terms...typically science fiction is considered a subset of speculative fiction, and here I inverted a lot of things).