That is complete horseshit. He absolutely took direct issue with slavery and the treatment of blacks. Yes, he had some misguided ideas about sending them back to Africa, and he wasn't diehard anti-slavery the whole time, but it is very clear that he was anything but indifferent.
That was just the first step (limiting new states as being free, which got changed to allowing the states to choose to be free or slave). The Civil War was pretty much inevitable, and the border states is where it really started (the fighting that took place there is what led to war).
I have no idea why people think slavery would have just ended, let alone the ones that think it would have ended on its own in the timeframe of the Civil War. There is nothing that suggests that. If anything, it was actually looking to go the opposite direction as the South would have become even more powerful (the cotton gin would have paved the way for the South to put their slaves to work in textile factories, which the South would have been making as they had the money). Also, many in the South were intending to use slave labor to exploit the new territories. There was simply no reason the South would have voluntarily given up slavery anytime close to the Civil War. Hell, the North was really just starting to (slavery was still quite prevalent in the North even then). You even contradict yourself, the new states had just as much slavery, which is exactly what the indication was, that it was not going to just go away.
If you think that slavery would have just ended, actually go and look at how the Native Americans were treated, human rights (women, children, sick, mentally handicapped, etc), all the way up to the Civil Rights movement. They were all lingering issues of American society that were directly responsible for slavery that lasted for decades (in fact, roughly an entire century after the Civil War, where the issue of slavery was made to be clearly wrong).