Antifa use physical violence to confront fascism. They regard free speech not to encompass hate speech.
Silencing an opposing political opinion through violence is a definition of fascism.
Correct we call that terrorism when it is to further your own political, religious, social, racial, or environmental views.
Domestic terrorism: Violent, criminal acts committed by individuals and/or groups to further ideological goals stemming from domestic influences, such as those of a political, religious, social, racial, or environmental nature.
How correct they are about what they’re fighting is a separate question. As you have demonstrated, there are many different definitions of fascism. The Berlin Wall was described as the “anti-fascism protection wall” by the DDR. And, of course, “my opponent is literally Hitler” is widely dismissed as a melodramatic overstatement.
However, if we presuppose that they are correct about who they are targeting — sadly plausible, given how many genocides followed the one in WW2 — then to say that Antifa as a movement is itself “fascist” is like saying that the Allied powers in WW2 were “fascist” for violently opposing the “political opinion” of Herr Adolf H. and Signore Benito M.