It doesn't matter what they thought. The claim that Lincoln and the Northern states were fighting for the right of self-determination is obvious hooey. They were fighting to impose confiscatory tariffs on the Southern states, and nothing more.
Then why did the South unilaterally in their declarations of secession identify maintaining slavery as a primary reason?
It doesn't matter what they put in their declarations of secession. The bottom line is that Lincoln didn't invade the Confederacy to end slavery. He didn't give a damn about the slaves. He was a white supremacist. He invaded the Confederacy to impose the Morill tariff on the South. He said so himself.
Quotes from Abe Lincoln..
"Whenever I hear any one arguing for slavery I feel a strong impulse to see it tried on him personally." The Collected Works of Abraham Lincoln edited by Roy P. Basler, Volume VIII, "Speech to One Hundred Fortieth Indiana Regiment" (March 17, 1865), p. 361.
"What I do say is, that no man is good enough to govern another man, without that other's consent. I say this is the leading principle - the sheet anchor of American republicanism." The Collected Works of Abraham Lincoln edited by Roy P. Basler, Volume II, "Speech at Peoria, Illinois" (October 16, 1854), p. 266.
"We think slavery a great moral wrong, and while we do not claim the right to touch it where it exists, we wish to treat it as a wrong in the territories, where our votes will reach it." The Collected Works of Abraham Lincoln edited by Roy P. Basler, Volume IV, "Speech at New Haven, Connecticut" (March 6, 1860), p. 16.
"In 1841 you and I had together a tedious low-water trip, on a Steam Boat from Louisville to St. Louis. You may remember, as I well do, that from Louisville to the mouth of the Ohio there were, on board, ten or a dozen slaves, shackled together with irons. That sight was a continual torment to me; and I see something like it every time I touch the Ohio, or any other slave-border." The Collected Works of Abraham Lincoln edited by Roy P. Basler, Volume II, "Letter to Joshua F. Speed" (August 24, 1855), p. 320.
"I am naturally anti-slavery. If slavery is not wrong, nothing is wrong. I can not remember when I did not so think, and feel." The Collected Works of Abraham Lincoln edited by Roy P. Basler, Volume VII, "Letter to Albert G. Hodges" (April 4, 1864), p. 281.