"Irrelevant"?
Non Slave Owners in the South were innocent, but the North still stole their money, invaded their land, burned their homes, and lied about the reason they were there.
The Declarations Of Secession basically said:
"You want to label us "Slave States"? That's fine, but we "Slave States" are declaring our independence."
Then it goes on to discuss the way the Union has rejected the initial principles of the Constitution and waged war on the South.
Basically, it was a long-worded description of the North's actions of Piracy.
Sure, it mentioned slavery. But that was a legal practice. Slaves were Legally sold by Africa, and Legally bought by Europeans and Americans. (The North engaged in slavery for hundreds of years).
There's nothing legal about Pirates who use war ships to block the free trade of goods between sovereign nations.