The proud Southern Heritage of the Confederacy represented by the “Rebel flag” as you call it, most likely was a by-product of Reconstruction, more than a yearning for the days of the Confederacy.
If the whole of the South wasn’t united before that occupation period, and it likely wasn’t, the actions of the occupiers sealed the deal.
Shelby Foote says pretty much the same thing in his notes at the end of Vol. 1 of his trilogy.