The movie isn't really about that kind of stuff, it mainly uses the CSA taking over whole country as a way to have slavery still around, which is really what the main focus of the movie is. It's not really that deep or serious of a look at what would realistically would have happened if the Confederacy had actually won the war.If it's presenting itself as a faux documentary rather than pure fantasy, then that implies an obligation to depict its subject as authentically as a real documentary. If it's just making up random, bad history instead of using alternate history to provide insights into real historical patterns and processes, then it's just empty sensationalism and I have no interest in "going with it."
I mean, the single most basic thing about the Confederacy is that they wanted to be separate from the rest of the US. They saw themselves as a distinct culture and the Northern states as outsiders. So if someone doesn't get that they wanted to split from the US rather than conquering the whole thing, then they've misunderstood one of the most basic facts about the CSA and the reasons for the war, and that renders the entire exercise invalid from the start.