If you use the term, "Antifa," it almost always refers to that political movement...

As you point out, there isn't really a single movement. There is no organization. Note also that I used lower-case, as it is not a proper name.

As an anti-semite is against Jewish people, it does not refer to a specific group.

If there ever rises an organized "Antifa", I would have to evaluate all their stated aims and goals before claiming any affiliation. As it stands, there isn't one. That's been your entire point with Dope. So I am antifa/anti-fascist because there is no extra baggage (yet). It's just a statement of something I oppose. I also am pro-choice as that is universally understood (ref: abortion), but I am not a member of any group that has that as part of their mission statement. The closest would be that I contribute occasionally to Planned Parenthood, who is about health education and reproductive freedom. But pro-choice does not necessarily equate to that specific organization.