To my knowledge, that book and the accompanying series is less of a celebration of those African civilizations and more of a whirlwind tour about cultures that many viewers would be largely unaware of.

Plus, even if he is a massive African history fanboy or whatever, that doesn't make him a supporter of all of their values. There are plenty of Civil War historians who think the Confederate States and its soldiers was fighting for an unworthy cause, but are still passionate about the topic.

Many pre-modern civilizations used slavery. A few prisoners taken from their neighbors employed as domestic servants, field-hands or (if they had the skills) artisans here and there, that would eventually be freed and integrated into the mainstream population, that was pretty common.

Situations where the economy was dependent on slavery were rarer.

In the 1500s the Spanish and Portuguese began importing slaves to the new world, followed by the French and English. They soon created a Racial notion of slavery. Slaves were not just people at the bottom of the social pyramid, but were seen as fundamentally lesser form of humanity. The children of freed slaves in Rome became more Romans, the children of freed slaves in Colonial America were always an other.

The CSA was an attempted state which was founded on the principle of preserving racial slavery. They made no secret of this.