Indeed, if I understand correctly, during the Civil War, millions of slaves which had previously produced the world's greatest supply of cotton, switched over to producing food for Confederate military & civilians, freeing up equivalent numbers of white men to serve.
And Confederate armies themselves consisted to a large percentage (circa 25% I think) of slaves who performed every function you can think of, except actual fighting.
So there's no doubt that slaves were critical to the Confederate war effort, and Lincoln's Emancipation Proclamation struck a blow at the heart of Slave Power.
But the key point to understand here is that Emancipation struck simultaneously in three different areas:
Rebels sometimes looked like scare crows and were scrawny but they always had plenty of ammo.