Well, he wasn’t talking about the Cold War at large, or even communism. If we can understand parts of the genre pastiche are westerns and others jidaigeki, then it shouldn’t be too hard to understand mixing and matching historical themes as well. And again, the quote isn’t even really about intention, Lucas and Cameron are discussing “terrorism” and its relationship to hegemony via Star Wars. Viet Cong can be swapped out for any group of rebels in history, Lucas is just talking about what it was in the 70s (when Star Wars was being made and released). The important part is that the Empire is the hegemonic power, and again you can swap that out with any in history whether it’s the British, the Nazis, America, etc. Star Wars is a simple story, not a sociopolitical treatise.

For Lucas, it’s just valent to his prequels (and the context of the 70s) that his Empire is America. It’s a reflection of Star Wars’ contemporaneous moment. One of the most ‘revolutionary’ times in pop art and music.