I'd argue that ignoring that any forced, unpaid labor under threat of violence is slavery is worse than "minimizing chattel slavery," full stop.

This is unintentionally drinking the corporate prison Kool Aid at best, and actively sanitizing our prison's cruel labor system at worst.

Accurately calling prison labor slavery isn't a knock on chattel slavery, it's an acknowledgement that it's changed. Say it's not as bad all you want, but it's still the same forces at work.