That's True.
The point here is that Lincoln's audience was Illinois people, NOT the people of South Carolina. And lets not forget that Steven Douglas was saying the same things during these debates, and won the Senatorial election. In fact Douglas was more of a neutral on Slavery and had a track record of being a party to writting the 1850 Missouri Compromise and Kansas-Nebraska act legislation.
Another little point is that Lincoln never campaigned in the South. He did not win any southern states. So why the proslavery talk? By the time of his 1st Inaugration speech most of the southern states had already seceeded and he wasn't really doing anything to win those states back in the Union. All of his proslavery talk was directed primarily to northern Audiences.
So in my opinion it is not as revealing WHAT Lincoln, or any other politician said, as it is to WHOM he was saying it to, and why.