...I am agreeing they mention slavery, that's why the Confederate states seceded, they didn't want the federal government interfering with their right to own slaves and run their economies using them. For Lincoln however he was both being "smart" in not attacking slavery directly because he knew if he alienated his supporters in those states he would be making a strategic error, and also because he didn't think he could actually do anything about it as president. At the time when Lincoln gave the Emancipation Proclamation, the people who would sympathize with that message were far ahead of him in recognizing and adopting emancipation as a moral justification for the war. Lincoln basically said, if you are fighting this war for freedom and liberty, join and fight for it. The error we make looking back is emphasizing this speech as the turning point, it was actually reacting to what abolitionists, slaves, and former slaves had already done.

I shared an excellent hour and a half interview with civil war historian Barbara Fields in another comment expressing this sentiment, often reciting from books and historical letters throughout, that gets deep into this topic. Obviously people are downvoting it, but she explains it clearly:

“it was the battle for emancipation and the people who pushed it forward… it was they who ennobled what otherwise would have been meaningless carnage into something higher. When a black solder in New Orleans said “liberty must take the day nothing shorter” he said in effect that when we count out those who have died and survey the carnage is must be for something higher than Union and free navigation of the Mississippi River”

I think what most people are trying to get at here was that Lincoln himself was not particularly a pro abolishinist. He was a lawyer who just wanted the Union to stay together and follow the current laws.

He was up against difficulty when he wanted new states to not allow slavery. This made the southern states mad, etc, etc, war. Even still at first, he did not free slaves. It wasn't until the war was underway and not going as well as hoped that freeing slaves became a thing. This was after a southern slave commandeered a southern ship and escaped to the north with it. A general then had to decide if they were required to "return property" or free the slave. He freed the slave, stating he had no obligation to "return property" to a force that was an enemy. This was a big decision at the time. I think that event set the ball rolling on freeing slaves.

So people are being pedantic. Yes it was about slavery. No, it was not (at first) about freeing slaves. That came later.