The writer was a bit more than disingenuous in their analysis.
One can not simply write about slavery or economic systems independently. In this case the Southern Economic System was predicated solely on the institution of slavery. In other words one can not talk about one without the other. Like taking one leg of a triangle away .... it is no longer a triangle.
However in an analysis of actual real reasons for session, Southern legislatures provided an insight to the reasons. First and foremost was the Southern Economic System, and second was States Rights, and third was the election of an abolitionist, Pres A Lincoln.
Of course on the converse side for cause of hostilities, first and foremost was the Constitutional issue of dissolution of the Union. One can not simply write that we are shooting at you because we want you to end slavery, but we can if there is an overriding legal principle, fundamental to the Union. (Note the similarity to the talk of Trump supporters, who believe they can at will overthrow the government simply because they don't like America ... sounds dangerously close to what Southerners were saying in 1860). Once the principle was invoked, secondary issues could and did come into play, even if political in nature.