Is “slave labor” in your post a euphemism for low paid work? Or how exactly is Microsoft or any other using slaves?

Slave: a person who is forced to work for and obey another and is considered to be their property; an enslaved person.

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."