The southeners dislike of Douglas's policy was still rooted in U.S. Supreme Court rulings, U.S. federal Laws, and the U.S. Constitution from which these laws and rulings stemed from. As demonstrated by the election of Lincoln, the Slave states did not have the political power to change anything. But to change the laws "legally" the Constitution had to be change, hence the 13th amendment. That was made very unlikely by the 1820 Missouri Compromise provisions until those were struck down by the Dred Scott ruling.
Yes, It was about Slavery, but it wasn't about slavery. lt was only about slavery as it pertained to the rule of law and equal protection of all citizens rights by the United States Government not only in the States, but in the territories also. The 1860 election wasn't about secession. But, it was about Constitutional Governance and the right to the protections guaranteed under that Constitution.
When Constitutional protections are voided what recourse is there left to the people? So in that way, the 1860 election was about secession. For example South Carolina's declaration to seceed, if Lincoln won that election. Why did South Carolina make such a declaration? Because they felt that Lincoln's election would insure the abrogation the rule of law and void those Constitutional protections.
The power of the Constitution of this country is that, if properly enforced, it gives the most protection to those who are the minority, or hold to the least popular practices, or ideals.