2 points
People are just nitpicking the meaning of the word Organization. Antifa is an organization in a very loose definition of the word. If you want to be more accurate, you’d call it a Network. Organizations (in the stricter sense) has a single leader and has a very tree-like structure with more power on top (like Corporations!), which Antifa obviously is not.
Though you’re correct in that Antifa is a “movement”.