I'll just leave this here, where George Lucas explicitly confirms the rebels were the communist Vietnamese and the Empire is america: https://youtu.be/Nxl3IoHKQ8c?t=56
And he explicitly corrects the interviewer when they attempt to call them the "English Empire" instead.
Lucas brings up the American Revolutionary War (he said something about "hayseeds in coonskin hats") as a parallel to the Vietnam War, probably to smooth over the discomfort the lib interviewer has with characterizing the modern and 60's US as the evil empire. So I don't think that's a correction, more like an insistence on continuing the parallel.
Side note, that turn of phrase was a masterful way to prevent/delay the interviewer from dismissing the viet kong as "terrorists." Lucas is extremely sharp. I definitely couldn't have directed the conversation as effectively.
Yeah he's absolutely trying to make sure there that it is explicitly understood that is the American empire not just an "evil" empire that you can swap to anyone else.
Ho Chi Minh famously wrote a letter to Truman asking for American support in their war against the French because they were trying to do the same thing the US in their revolution. he believed the propaganda and got bitten by the reality - Truman never replied.
IIRC he said they had greater freedom in what they could make because they weren't pushed to produce as much profit as possible like he was, even if they needed to steer clear of censors. It makes a lot of sense, he is from the era where the soviet filmmakers were putting out some excellent films. And obv because capitalism kills art.