The Confederacy was fighting against the United States. Under tankie logic, that must mean that they’re virtuous and attempting to build communism

Lol.
What we think is that slavery and colonialism is bad, and that, if forced to pick a side, it’s the side whose interests align more with those of the workers of the world.
Also, rather funny to pretend that even the basic uncompromising “anti-US” stance would lead to siding with the US but with more slavery.