This plan might be many things, but slavery ain't one of 'em. I didn't see anything that said he unpaid work was compulsory.
Unpaid volunteer work and internships are a great way for young folks to gain experience and increase their appeal to future employers.
I'm very thankful I avoided internships. I believe in work and I believe in charity. I work for money. I like my field and often have projects I enjoy, but I work for money. No money, no work. I've always been very skeptical of unpaid internships, especially as they frequently venture into illegal territory. Volunteer work is different, in my opinion. I do it for altruism, and expect/want no compensation. Generally it's to help an individual, improve the community, for the public good, whatever. Volunteering to make someone else money is not altruism, in my opinion. Not claiming that folks shouldn't be allowed to freely contract as they want. Stupid isn't and probably shouldn't be illegal. But doing mandatory unpaid work is the definition of slavery. Working off a sentence isn't the same as "unpaid", ditto voluntary indentured servitude, etc.
That said, it's a bloody think tank study from a place with a really bad web site. I wouldn't flip excrement over the matter.
http://www.kepe.gr/EN_Pages/home.htm