I'm really just channeling from memory Luxemburg *Call to the Workers of the World" and Lenin's Imperialism: Highest Stage of Capitalism
The film kind of portrays it, or hints at it: the terms of the peace treaty were portrayed as not just humiliating but impoverishing.
e: they also hinted that the social democrats felt a pressure to acquiesce to the armistice to prevent "Bolshevism" from taking over since the war had exhausted the legitimacy of the imperial government and the successor government if peace didn't follow soon. Keeping capitalism alive meant to accept severe loss to prevent total loss to the communists in Germany and throughout Europe.