I think the idea there relates to his recent $1,000,000 donation to prison reform charities. I'm only reading/learning about this now, but theres an exception written into the 13th amendment that essentially allows slavery if its punishment for a crime. So I guess what he's getting at is that racial inequality in the prison system is possibly caused by the 13th amendment, and that removing it might help that? Considering Kim's meeting with the president (and I think shes becoming a lawyer to work on these sorts of cases?), this seems to be a bit of a theme with their philanthropy recently.
To me the whole slavery is a choice thing was taken way out of context and blown up when it didn't need to be, I don't see much of a relation to these 13th amendment lines. I really don't think hes trying to say the 13th amendment is unnecessary because 'slavery was a choice' any or something like that.