For example many CPSU members were formally members of the social democratic party, almost all Eastern Bloc communist parties were mergers of the preexisting communist parties with social democratic parties, and the Korean Social Democratic Party is one of the few opposition parties in the DPRK
That claim was made by Comrade Stalin in 1924, a decade before the rise of fascism in Germany. The assessment turned out to be unhelpful and Stalin himself abandoned this theory. In 1935 the Comintern changed from the “social fascism” to the “popular front” theory.
Did the social democrats not fight against fascism hard enough? Yes. Are they our political enemy because they support capitalism? In the grand narrative yes, but for specific goals in the near future inside bourgeois states, likely not. Does that make them fascists? No. If supporting capitalism makes them fascists, any other ideology besides communism would be fascist and the word would lose its meaning.
Pontificating about the true nature of one group or another is idealism. Look at the material goal you have at hand, the concrete thing your org is trying to accomplish right now, and see who will be for or against you. It is idealism to do otherwise. Just because social democrats in general don’t want to kill landlords does not mean specific people who call themselves socdems won’t join a tenants union. You judge people individually, you assess their motives and resolve individually, you do the same for specific organiztions, and you see how they fall in terms of power and alignment for the specific goal you have.