I mean,
we can say that and be done with the subject. For instance, just look at the seceding states' articles of secession: they quite plainly spell out that they are seceding because of slavery and to join the other slaveholding states.
But there are a whole lot of Americans who wish to believe that the South had a noble cause (https://en.wikipedia.org/wiki/Lost_Cause_of_the_Confederacy), and they are more than happy to buy books and watch TV shows explaining what that cause is, so such books and TV shows get written.
The Civil War was just a couple of generations ago, and involved a huge chunk of the country. The various organizations attempting to rehabilitate their legacy (e.g., the Daughters of the Confederacy) are younger still, and active today. It's not actually surprising that there are a large number of Americans who for whatever reason wish to believe that there was a better reason for the Confederacy to do what it did than slavery - the Daughters, for instance, are quite literally descended from Confederate soldiers, so there is a natural desire to believe that they fought for a worthy cause. But the number of people who believe it is hardly evidence that it's true.
What is noteworthy that Paul Graham (who wasn't even born in the US!) is one of those people, and moreover doesn't simply believe (as you're claiming) that there's a legitimate debate about what the Civil War was about - he believes that the possibility that the Civil War was about slavery is a "cartoon version" which can't possibly be true.
And yes, it's a throwaway line in an unrelated talk. But he still said it as if he believed it, and the alternative interpretation - that he doesn't really believe it and said it anyway because he thought it would please his audience - is that he is a man of poor judgment and a loud mouth in things he should judge less and speak less on, which goes to my point that Sam did not "bro up" YC, it was already rotten.