If hard labor is the punishment, and the punishment is commensurate with the crime, then no, it's not slavery. Slavery obtains under different conditions.
Also, the word "except" is our key hint here.
I seriously doubt (and see no evidence for the idea) that the US is so lawless that we can have almost a quarter of all prisoners in the entire world while only having a fraction of its population.
This speaks strongly to the idea that huge amounts of wrongful imprisonment occurs and (as a logical consequence) these people are slaves against their will without their having done anything worthy of punishment.
The difference is solely the standing law. During the nazi regimen (godwin's law, but really) many of the appalling things they did were legal. It was legal to execute, and it was legal to do forced labor.
Slavery was always legal, that was the problem all along! A slave could not physically escape without the fear of unpunished retribution.
Redefining the word so that prison slavery doesn't qualify seems obtuse, counterproductive, and doesn't even have the defense of being historical.
If locking people up and forcing them to work is making people billions, it's slavery. Period.