my point really is less concerned with the individual mindsets of those that joined the confederacy vs the reason the confederacy was a thing to begin with, but i can agree that what you say was no doubt commonplace. if we're discussing why the war was a war, we can't nonchalantly glaze over slavery, whether its being framed as a religiously-ordained institution or an economic tool. it was a primary cause. and whether the small-time farmers who enlisted in 1861 knew it at the time or not, they almost certainly must have come to learn it over time.
re: the argument that non-slave owners wouldn't have been concerned with the institution of slavery, we have to be aware that even to this day, large factions of people support legislation aimed at helping people above them in the socioeconomic ladder and do very little for themselves (trickle-down economics). The "whats best for us will done day be best for you" sermon is as old as civilized society itself.