I think his point is that slavery is not outlawed by the 13th amendment as most people assume (even the Google AI summary reads: "The 13th Amendment to the United States Constitution, ratified in 1865, officially abolished slavery and involuntary servitude in the United States.").

However, if you actually read it, the 13th amendment makes an explicit allowance for slavery (i.e. expressly allows it):

"Neither slavery nor involuntary servitude, *except as a punishment for crime whereof the party shall have been duly convicted*" (emphasis mine obviously since Markdown didn't exist in 1865)