Slave labor didn't stop being integral element after the Civil War. It was scaled back, but it's still both locally an integral element of the economies of many states (via prison labor, to say nothing of how under-the-table migrant dealings go) and via imperialism, etc. used abroad.

I'm not attacking Smith. The "invisible hand" thing is silly and short-sighted, but his work more broadly was the foundation for Marx economically. I'm attacking capitalism as it has existed in history, where it has virtually always used slave labor as an integral element.