They are not against "fascism" as we commonly think of it (e.g., Mussolini's fascists). They are opposed to modern capitalism, and they call it "fascism," just like the Antifaschistische Aktion movement of 1930's Germany.
Antifa "view[s] fascism primarily as the final stage of capitalism rather than as a specific movement or group and therefore applied the term to all other parties [all of their opposition, regardless of whether they were actually fascists]"
Link: https://en.wikipedia.org/wiki/Antifaschistische_Aktion