It's also time-bound for the length of the sentence. So like sure it's slavery...temporarily, non-inherited, non-race-specific, as a punishment for a crime, at least sometimes paid.

Which is just a lot of caveats.

Similarly, having a job is just temporary, non-inherited, non-race-specific paid slavery where you get to pick your slave master. Sure you can make that argument but it's not a very good one.

A lot of stuff about the US prison system is really bad, including this part, it's just not literally slavery, and it doesn't have to be slavery to be really bad and need changing.