The irony is that the original antifa - https://en.wikipedia.org/wiki/Antifaschistische_Aktion - was, essentially, a KPD-affiliated militia. As such, their definition of "fascism" included social democrats at some point, and their propaganda included bits like "fighting fascism means fighting the SPD just as much as it means fighting Hitler and the parties of Brüning".

I don't know why anti-authoritarian lefties today are so eager to embrace this legacy, or at least its symbolism.