It’s worth the effort.
I suggest you read Pryamids first, not because it’s particularly representative of the series, but to get it out of the way so you can grok all the “pTerry” nonsense that people seem to be so fond of.
I strongly suspect that, as an academic matter, the artists involved, whether as singer or songwriter, absolutely recognized that slaves were freed as a result of the Civil War, but doubtlessly they also realized that a great many black men (among others) were dying in their own time from a needless war that was in fact bound to end by accomplishing nothing (for the US and the Americans who died there, at least).
But, as often happens with art, they chose to sacrifice literal truth for narrative simplicity and contemporary relevance. Story Truth (a particular message) over literal truth (mere fact).
Uhh…Cat Stevens?
No, that’s “Yusuf’s in the Cradle.”