At the link I gave: "In the usage of the Soviet Union, the Comintern and its affiliated parties, including the KPD, the epithet fascist was used from the 1920s to describe capitalist society in general and virtually any anti-Soviet or anti-Stalinist activity or opinion. ... In KPD and Soviet usage, fascism was primarily viewed as the final stage of capitalism rather than a specific group or movement such as the Italian Fascists or the German Nazis and based on this theory the term was applied very broadly."

That makes sense, given what they truly oppose in the US. Many Americans are thinking "facism" is a mean people movement, like Mussolini's thugs, or the KKK. But, they (Antifa) are thinking of fascism as "American Capitalism as it exists now." Obviously they oppose the latter, and they are willing to be mean people to get there.