Their narrative glossed over slavery, and focused on the other aspects of the conflict: the industrialized north, with its big banks and big corporations, versus the agrarian south. Do you remember the scene in Hamilton where they negotiate about where to put the capital? And the Hamilton says something along the lines of “let the south have the capital, we’ll have the bank where the real power will be.” It’s not hard to make the south look like the good guys if you ignore the Confederate elites’ actual reasons for the war.
I grew up partly in the South of the civil rights era and went to one de-facto segregated school. (Dad was military, stationed in various places.) I was a kid, but I paid attention to the news, and my impression was that racism was the clear, undeniable subtext to the "being a rebel" part: Many Southerners — notably, most of those who held political- and economic power — resented federal "interference" with "Southern ways," namely the violent oppression of blacks to keep them as an impoverished, powerless, underclass workforce.
(In 2020 I read a tweet thread from an anonymous Army officer, recapping the history of post-Civil War Reconstruction and the Union's de facto surrender in 1877; the quote that stuck with me was, "Sherman should've mowed the deep south like a lawn, making multiple passes")
Never got that one. Nothing like the flag of a breakaway state founded to preserve chattel slavery as a symbol of… anti-authoritarianism?