I think that was more true in the lowland south where slavery was a pervasive part of the culture and the oligarchs could stir up fears. I'm not sure if that was a strong factor in the many highland areas of the south. There was a lot of people in those areas which had little contact with slavery that had other reasons to fight. Some fought due to state loyalty, some fought to keep an army from afar away from their home territory and many fought from being compelled by the heavy hand of the confederate government.
Where I cannot acquiesce is in the causation.
The states of SC & Georgia sent out what were called "Commissioners" to lobby the state legislations of the remaining Southern states that would eventually make up the Confederacy. The speeches and letters they wrote clearly laid out the preservation of slavery as the main cause for secession.
The Deceleration's of Secession from the Confederate States all lay out, in no uncertain terms, the reason for secession was slavery.
The debate on the motivations of the individual soldier bother me only insofar as they are extrapolated as an acquittal and denial of the actual cause of the war on a larger scale. I realize the average soldier, on both sides, did not fight over slaves directly. Nevertheless, that was the reason they were forced to pick up arms to defend their other reasons.