It's a fucking hashtag (and the spiritual predecessors of hastags like email label, etc for the years prior). Anyone organizing or "promoting" an event (demonstration, protest, whatever) would label it antifa if they felt that the people who were strongly opposed to fascism would find it worthwhile to support the cause.
Yes there are people out there who primarily focused on supporting anything that opposed fascism, and weirdly some of them know each other. (Strange things happen when people congregate, some of them talk to each other and even form friendships... shocking I know).
The idea that its anything other than people in a scene knowing each other is just as dumb as people saying OSS is a terrorist organization because Anonymous released loic under a public domain license.