Slavery was a matter of state law. As you know, there are self-proclaimed libertarians on FR who advocate gutting the 13th Amendment and allowing the ownership of human beings.
Nice dodge... but the US Constitution guaranteed the right to due process, and the right to free speech, and freedom of religion, and freedom from unreasonable search and siezure, and the right to keep and bear arms, and all manner of other protections for men.
Just not the black men the founders happened to own.
That would have been inconvenient.
Your earlier ridiculed another for suggesting that the founders may have often been untrue to the intent of the constitution. Sadly they were.... They were oftentimes hypocrites. You may attempt to rationalize otherwise, but that doesn't change the truth of it.