Can you understand his anger at such self-serving excuses and evasions, at the waving away of a century of conflicts and problems like they didn't exist? Can you see that he might have some reason to be angered or saddened by such a brushing under the carpet of some of the hard realities of American history?
Of course, we don't know what America would have looked like had the Confederates won. And the filmmakers do exaggerate things for effect. They're not writing a thesis or making some mathematical model of an alternative universe. They're not trying to be fair-minded above all else. They're using a certain amount of absurdity to point out the absurdities in another point of view.
Most people who know the history will likely leave the theatre recognizing that the movie exaggerates and isn't entirely fair, but perhaps they'll question some of the assumptions of the neoconfederate propaganda of recent years. We can recognize the absurdity and exaggeration, but also see the point. By contrast, some of today's Confederate propaganda is absurd, but pretends to be true. I don't know if the film works or not, but good satire can have a cleansing effect, but deflating some of the bad arguments that come to predominate in public controversies.