Antifa groups tend to scorn Democrats as “neolib centrist sellouts” etc.
The fascists are explicitly aligned with the leader of the Republican Party.
The KPD was an actual force in German politics. It contested & won elections. It was a national party tied to a centralized international movement.
That just cannot be said of Antifa.
It was that right-wing parties & the reactionary Weimar political establishment refused to accept the Social Democratic Party as a legitimate political actor, despite its winning the most votes in elections.