As others have said, the rehabilitation aspect is dubious. It depends on what the person "experiences" for that length of time. If there's therapy in time-dilation-space then sure go right ahead and sign me up as well. I'll just Goku-it up in my chamber of time and space and work some shit out in time for my morning shit. But you and I both know it ain't going to work that way. Prisoners will just be trapped in an empty void with only their own thoughts to keep them company, most likely rendering them insane. An infinite solitary confinement is just plain torture.
Edit: so I googled the article and its laughable how easily the author slides right in to dystopian fanfic. "This would, obviously, be much cheaper for the taxpayer than extending criminals’ lifespans to enable them to serve 1,000 years in real time." Obviously.
One day Steve said: You know what? Keeping a prisoner a life for a thousand years is fucking expensive. What if we didn't have to?
And from that conversation our company was born. Little did we know that death sentence is still a thing or that humans don't even live that long. But boy did we scam some investors.