No, it really wasn't. That's nothing more than a whitewashing of history. They were very clear and straightforward about what they were doing in literally every single fucking thing that they did or said. The ... recalibration ... of their positions came after the war was already over, not when they were actually going to war for the sake of continuing slavery.
Succession was the byproduct of a feeling that slavery would be curtailed if they stayed within the US, it wasn't their primary motivation. There's a reason that the Skaveholding States seceded and the Freedom Loving States or whatever weren't a name that anyone proudly called themselves.
You should read history a little closer. While it is true that the States seceded because they were afraid that new territories being admitted as States could add more congressional power to the growing abolitionist movement, not all slave States seceded. At the time the war started there were four slave States that remained in the Union (Maryland, Missouri, Delaware, and Kentucky), plus slavery was allowed Washington D.C.
The war was about the right of States to secede (to save the Union), as clearly stated by Lincoln in his letter to Horace Greeley (a staunch Republican abolitionist) in response Greeley's strong editorial insisting that Lincoln free the slaves.
Lindoln's letter to Horace Greeley
If there be those who would not save the Union, unless they could at the same time save slavery, I do not agree with them. If there be those who would not save the Union unless they could at the same time destroy slavery, I do not agree with them. My paramount object in this struggle is to save the Union, and is not either to save or to destroy slavery. If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union.... I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men everywhere could be free
The Emancipation Proclamation was not issued until almost three years into the war and did not free any slaves in even Tennessee because it was at that time already occupied by the Union. It only freed slaves in the "States in rebellion".