Yes, this is the opinion of one of the minority, a wealthy slave owner. Can it be demonstrated that the non-slave owners embraced these sentiments, and embraced them strongly enough to risk life or crippling wounds during wartime so that they could remain in that "superior" position? Was that worth dying for to them?
I think the question here is - if not that, then what else?
There are thus these possibilities:
1) The non slave owners (i.e. the lower class, with slave owners forming an oft substantial middle class - in some states practically half of white families owned at least one slave so
non slave owners were only a hair off being the minority!) did not believe in slavery, but signed up to fight and die for a state avowedly arranged for slavery for some other reason.
1a) That reason was state patriotism, and these people identified with their state more strongly than their country during a period of intense North-South culture war in which the dividing line was the presence or absence of slavery.
1b) That reason was something other than state patriotism, and they believed it despite the wealthy elites not even really mentioning it.
2) The non slave owners did believe in slavery or were in some way invested in it, to the extent that they were willing to fight or die for it.
2a) They considered slavery to be good despite not having a hope of at some point joining the slave-owning group, and this was because they felt it was better for blacks to be "down".
2b) They considered slavery to be good because they had a hope of joining the slave-owning group.
2c) They considered slavery to be good because it was divinely supported; i.e. that it was morally right.
One useful bit of information is the 1860 election. While obviously most Southern states didn't bother to put Lincoln on the ballot, Virginia did, and Lincoln got effectively zero votes - but Douglas ("popular sovereignty" and the rejection of the pro-slavery Charleston platform) got far fewer than Breckinridge (which was a pro-slavery platform) and Bell (who ignored slavery entirely); in the other Southern states Douglas was handily beaten by Breckinridge, and the only competition for Breckinridge was Bell.
I would say that heavily voting for the "pro slavery Democrat" over the "popular sovereignty Democrat" given the choice between the two would indicate that in the South there was genuine
support for slavery as a positive good, including among the lower classes (since, obviously, you can't win an election with just the upper classes - though you can win with a platform which is
in favour of the upper classes if you can get at least some segment of the lower classes to buy into it.)