Slavery was the primary issue of "states rights." Southern politicians certainly made slavery the centerpiece of their speeches, as did northern politicians. If you read letters written by northern soldiers to their families, slavery was very much the sin against which they were fighting.I think someone at History.com needs to study history. Slavery didn't divide the country, state's rights did. Lincoln himself said that if he could hold the union together and not free one slave, he'd do it. In reality, he signed the emancipation proclamation in 1863 to keep the British, who were very much anti slavery, from aiding the South. Making slavery an issue was a political move.
I see they actually did see fit to admit in passing that the civil war was not over slavery.
You're right, though, that the primary effect of the Emancipation Proclamation was to keep the British out of the war.