Modern Antifa roots are not in the Weimar Republic Antifaschisitsche Aktion (1919-1933), but in the violent leftist & terrorist organizations of West Germany, in the 1960-70s. The Weimar Republic anti-fascists were destroyed twice, first by Hitler and then by Stalin.
The Nazi regime, once it came too power, eliminated Socialists and Communists and their militias. Surviving anti-fascists attempted to reorganize in 1945, after the fall of Nazism in the Soviet zone of occupation. Old German Communists and even Socialists probably thought of the Soviet Union as a friend. Stalin let them organize just enough to reveal themselves, and then destroyed them, again. From the horse’s mouth – Jacobin Magazine, 2017:
“Following their collapse in late 1945 and early 1946, Antifas would disappear from the German political stage for nearly four decades. The modern Antifa with which most people associate the term has no practical historical connection to the movement from which it takes its name, but is instead a product of West Germany’s squatter scene and autonomist movement in the 1980s — itself a unique outgrowth of 1968 considerably less oriented towards the industrial working class than its Italian counterpart.” (emphasis is added)
Modern Antifa is not related to those extinct anti-fascist organizations. Instead, Antifa stems from 1960s terrorist organizations, such as the Baader-Meinhof gang, also known as Red Army Faction (RAF). See Antifa is heir to Germany’s Red Army Faction, daughter of terrorist Ulrike Meinhof says. In 1976, RAF and Popular Front for the Liberation of Palestine (PFLP) hijacked an airliner flying from France to Israel and took its Jewish passengers hostage. The passengers were later liberated in the Entebbe raid.
P.S. There is also obvious differences in the operational principles: Antifaschisitsche Aktion organized openly, as party militias of the Weimar Republic did. The Antifa of today organizes covertly, like gangs and terrorist groups do.