I think this has more truth than many of the claims here. It's obvious that slavery couldn't be the entire issue, though, as slaveholding still occurred in the Union. It was an invasion of the Confederacy, not an invasion of slaveholding areas.
It's very difficult to imagine the Civil War occurring
without the issue of slavery, the problem of which had been explicitly threatening the integrity of Union since the Union itself began.
The sectional crises were of long standing. The Missouri Compromise of 1820, the Compromise of 1850, the Kansas-Nebraska Act of 1854 all led in one way or another to the Civil War -- and the Kansas-Nebraska Act actually resulted in a war.
Moreover, the political climate was trending toward abolition of slavery: it was the primary issue that resulted in the formation of the Republican Party, for example, and political trends pointed to eventual abolitionist majorities in the House and Senate. The numbers of potential Free states above the Mason-Dixon Line would have permitted Constitutional Amendments to that effect regardless of what the slave states wanted.
That's why the Southern states began seceding when Lincoln was elected: the handwriting was on the wall. Stay in the Union and slavery would be abolished - the only alternative was to leave the Union, which they did.
The question facing Lincoln was not what to do about slavery, but rather, what to do about secession. While it may or may not have been part of his reasoning, the fact is that a successful and even uncontested secession would have led to war between Free and Slave states regardless.