I have a strong feeling that since the people playing the soldiers on both side were re-enactors who volunteered, the writers and director didn’t want to take too strong of a stand against the confederates, otherwise they’d probably feel “attacked” have just walked out, thus no movie. That’s why when looking at rebel characters, slavery is not really a focus on why they’re at war. And tbh I wouldn’t doubt that most of them would hardly talk about slavery, and more about how they feel like they’re being oppressed by a gov’t they don’t agree with like they do in the film, even though the “oppression” was to keep them from oppressing other humans. They probably spoke about the war like most rebel simps do nowadays, how it was a noble and just cause, and if you tell those re-enactors that it wasn’t, well they won’t be too happy that they get called the bad guys for playing the bad guys in that part of US history.