So, ignoring all the other stuff you got wrong and ignored and hyper focusing on a technicality, got it.

There is a significant difference from the systemic form of chattel slavery used in the US pre-civil war and the use of forced labor in prisons. Like at that point you might as well argue that PoC don't have a particular claim to have been oppressed because white people go to prison sometimes which means that they've technically been enslaved too.

The Civil War still liberated a ton of people, even if the system that followed was far from perfect. Likewise, when the Russian and Chinese revolutions ended serfdom, it was a major step in the right direction - even though Western "leftists" will never forgive them for it.