I don't even think this is what they were talking about but there are over one million prisoners in the US and prisoners can be forced to work without pay. The constitution doesn't consider that slavery but the rest of the world certainly can.
It does actually.
Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Section 1 of the 13th Amendment explicitly refers to this as slavery:
“Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.”
https://en.m.wikipedia.org/wiki/Thirteenth_Amendment_to_the_...
I also don't have a reference on hand. I listened to an NPR radio segment that blew my mind. There's a bunch of information from reputable sources but finding a specific number can be tricky. First of all, it's estimated because it's obviously unknown. Also, defining the "bounds" of slavery inherently changes the count. There is a fairly recent citation that states there are 400,000 enslaved people in the United States on any given day.
https://www.state.gov/what-is-modern-slavery/
https://en.wikipedia.org/wiki/Human_trafficking_in_the_Unite...
https://www.cbp.gov/border-security/human-trafficking
https://www.theguardian.com/world/2018/jul/19/us-modern-slav...