That particular line struck me as very odd, especially considering no other characters question it. I suppose he could be lumping Nazis and Soviets together as enemies of democracy or liberty. The only other times I have seen the two be considered one-of-the-same are when I've read proponents of 'horseshoe theory' and anti-Semitic conspiracy theories of the Jews being responsible for both the Nazis and the Soviet Union.
Having never seen the movie, was the spy he was helping East German? That could be the connection. There were also some Eastern Europeans that were forced to fight the Atlantic wall. I can think of one case in particular at Omaha where there were Poles fighting with the Germans.