How very quaint—and patently false.
The Emancipation Proclamation —which freed all the slaves—was issued on January 1, 1863 (during the War). So how did Slavery become the scapegoat after the War again? Seems like it was a pretty important cause during the Civil War—and indeed before it.
The notion that slavery was made into a scapegoat after the war is simply ridiculous on its face. Indeed, the Abolition of Slavery was heavily used as a "righteous cause" to justify the Northern prosecution of the Civil War—and to convince Northern mothers to send their sons off to fight it.
Thus, the idea that the Civil War primarily "was fought on the grounds of tariffs imposed by DC" is the veritable height of revisionist pro-Confederate propaganda, and cannot be effectively argued by any serious Civil War authority...
The Emancipation Proclamation only freed slaves in areas (states) rebelling against the Federal government. Slaves in slave states loyal to the Federal government remained in their status for example Delaware. (Yes Delaware was a slave state! Slave population in Delaware was something like 120 slaves.) It took the 13th amendment(Ratified 12/06/1865) to abolish slavery everywhere in the US.