I would guess it is being downvoted because while what it says about the 13th Amendment is correct it isn't really relevant to the question it was answering.
The question was whether or not US prisons use slavery. He answered the question of whether or not it would be legal for US prisons to use slavery. While is it legal, it is not mandated.
A proper answer would examine the labor requirements actually in use in US prisons, compare them to labor requirements in other first world country prisons (and yes, several other first world countries make prisoners work), define just what they mean by slavery, and then try to make the case that the differences between what the US does and what other first world countries with required prison labor do is enough to make it slavery in the US.
The question was whether or not US prisons use slavery. He answered the question of whether or not it would be legal for US prisons to use slavery. While is it legal, it is not mandated.
A proper answer would examine the labor requirements actually in use in US prisons, compare them to labor requirements in other first world country prisons (and yes, several other first world countries make prisoners work), define just what they mean by slavery, and then try to make the case that the differences between what the US does and what other first world countries with required prison labor do is enough to make it slavery in the US.