That article is written by someone who is not a historian.
I ask you this, what did the south want freedom from?
To say that the Civil War was not about slavery is ignorant of the facts. Lincoln ran on a anti-slavery ticket. The whole group was running on anti-slavery. The tariffs were an issue, but not the cause of succession. You can tell this, by the fact that the south did not leave after they were passed, but instead, left after Lincoln won his election.
The historical context was that the north was largely anti-slavery and the south was not. There had been rising tensions over this issue, which is why the election was so important. Cali was a big state, that could tip the tide in the anti slavery movement. With Cali, the south was outnumbered and the 13th constitutional amendment could be passed. The victory of Lincoln was seen as the final nail in the slavery coffin. That is the reason the south left before Lincoln even took office, because it was assumed.
The nation saw a Lincoln win as the path to ending slavery. Lincoln also saw this, and knew it could very well spark a civil war. That is why he did not push for the 13th constitutional amendment, because he felt it would trigger a war. He sought to try and work out a deal with the south almost the entire war. He personally said that slavery was wrong and wanted it ended. He also knew that if it were simply ended, there would be a war. He figured a compromise that would eventually lead to the end of slavery would be best.
So, to say that the Civil war was not about slavery is wrong. There is a reason just about every historian that has researched the topic agrees that slavery was the key issue.