I presume that he meant that a under the 10th Amendment, slavery was not regulated by the Federal government. BUT, the 13th changed that. So it's not a 10th Amendment issue anymore, because the 13th gave that power to the Federal government.
But you'll never convince a self-described "libertarian" that their interpretation of the Constitution is wrong.
I would love to know what the Tenth Amendment has to do with the Thirteenth Amendment?
One is state rights to self-legislate and the other is slavery.
If you're willing I'm willing to debate both and show that both are entirely different ideas, not only that but I can elaborate on how and illustrate the background and reasoning behind the manifested ideas.