I agree that the Lost Cause myth is romantic, and I'd say that Whedon used it very effectively as a theme.
I can't really agree with Feral Historian's take that this myth was 'kinda true' for the south as that seems to suggest that southern fighters are somewhat absolved of guilt. "They were just trying to preserve their way of life!" When that life revolved around assisting plantations in maintaining control over their slave populations, often by hunting down slaves, or acting as overseers of their work, rings hollow to me.
It reads the same as anyone who's kept their head down to get by in an unjust system. You are culpable. And then fighting to try and preserve that unjust system makes you even more culpable.
Really not the right take to say "we lost the Election because of Muslims not voting for Kamala".