They are part right, if we really want to give them the benefit of the doubt. For the south it was absolutely about preserving slavery, but for the north abolishing it was still kind of a controversial topic.
The decision to make it about ending slavery from Lincoln's part was part tactical, even though he personally always wanted to do so anyway. It made a lot of former slaves and other black people available for enlistment and also secured the support of people opposing slavery.
But initially it was more about the southern paranoia of the north forcing them to abolish slavery and since the north could not provide any security about this, they decided to quit, which lead the north to try and preserve the union.
Lincoln believed that the Founding Fathers intended for slavery to eventually fade away to extinction. But for this to happen, there could be no further spread of slavery into new states. The Compromise of 1850 and the Kansas-Nebraska Act both steered slavery down a different path of proliferation.
Lincoln's policy during the Republican nomination and general election was to follow the path laid out by the Constitution. Meaning: honor the fugitive slave law and to make no infringements upon the South's right to slavery. However, Lincoln made it very clear that slavery will remain only where it currently was in place. There would be no further spreading of slavery into newly adopted states.
Most of my information comes from the book Team of Rivals by Doris Kearns Goodwin. I highly recommend it for anyone looking for a Lincoln biography.
There was an article I read a wile back so I may be misremembering. It claimed they the wealth of southern plantations was the slaves, the land and other assets were worth hardly anything. Many of these places had large amounts of debt tied to the value of their slaves. The fear was not just that the north would make slavery illegal, but that the actions being taken to limit slavery in new states would cause the price of slaves to drop and make all the rich slave owners broke.