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.
Silencing an opposing political opinion through violence is a definition of fascism.