It seems your definition of what constitutes "Science Fiction" is different than the rest of the world, and even the science fiction literary community. That's fine -- you can personally define it any way you want, but it really isn't realistic to say that The Expanse is not science fiction. The world of science fiction calls it science fiction, even if you don't. You could say that you don't think a dog-show-poodle is a dog because real dogs don't have their coats cut like a show-poodle --- but no matter how you personally feel about it, the dog world would still consider that poodle to be a dog.
Science fiction can still (and often does) take place in a world with deep social issues that does not have FTL space-faring capabilities nor is part of a galactic civilization.
And I'm not saying that you have to LIKE The Expanse; heavens knows there is plenty of science fiction (both on film and in books) that I personally do not enjoy. However, even if I don't like some works of science fiction, the works I don't like are still science fiction.
The lines blur pretty easily between Sci Fi and Fantasy, depending on who you ask. In my opinion The Expanse sits firmly in the Fantasy realm, while Star Trek is Science Fiction.