It is the principle of the Declaration of Independence, and yes, it eventually defeated slavery, but when it was initially written it was intended to ignore the condition known as slavery.
Slaves were suffering abuse.
Slaves thought your Confederacy was intolerable.
I'm sure they didn't think much of the slave owning US Confederacy either.
Slaves had no say in the Union at that time. It wasn't until they served the purpose of propaganda that they ever got a say in anything.
Pointing out the condition of slavery as an argument against independence merely argues against the Colonists right to have independence because they too were slaverholders at the time.
The point here is that you cannot impugn the legitimacy of Southern states gaining Independence without damaging the legitimacy of the original thirteen colonies gaining independence. Whatever applies to the Southern states, must also apply to the original colonies four score and seven years" earlier.
If slavery delegitimizes the Southern state's right to independence, then it must also delegitimize the original Colonies right to independence.
They too were a slave owning Confederacy seceding from a Union and led by a slave owning General from Virginia.