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.