No, it's not a euphemism for low pay work.

As the article describes, many of the laborers are forced to work and many are unpaid. They are punished with violence if they do not work: solitary confinement and parole rejection.

It's supported by the 13th amendment, which uses the same language ("slavery" being legal punishment for criminals): "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."