[int_19h]

That right belongs to the people, not the states. Which in slave states included the slaves. In two Confederate states, slaves were an outright majority, and in most of the rest, they were a large plurality. Do you seriously believe that Confederate politicians could be meaningfully described as representing "the people" dissolving political bonds, given that those politicians did not in any way represent all the slaves by design? It's about as meaningful as claiming that Spartan aristocracy represented helots or Messenians.

Not that it matters much. Even if it is democratic, there's no moral right to self-determination if that self-determination is explicitly for the purposes of brutal oppression of some minority.