I'm not convinced that there are indeed 27 million people fitting that definition at the moment, but I can't see how someone fitting that description is anything other than a slave. (Hmm, though I suppose legitimate prisoners can be forced to do work without being "slaves" so perhaps the definition does require tinkering around the edges.)
Modern slavery is an important issue (for humans in general, leaving aside its relevance to HN). When the word is misused, the issue may be trivialized, and I think it would work against solving the slavery problem.
Modern slavery is an important problem, it should not be trivialized. Trivialized is not the right word; I am looking for an English word... when one misapplies a word and thus presents the situation direr than it is, diluting the meaning of the word. Like what happens with the word "Nazi" in modern American English usage (soup nazi etc). However nazis are almost gone and slavery is not. And rigging the numbers does not help.
Citation needed? I wouldn't be surprised to learn that, under his definition, there are more "slaves" than ever before (just given the increasing size of population).
On the other hand, I agree with you that we need to be careful with the vocabulary. While I'm not trying to downplay how serious the problem is is, I'm not sure if this definition of slavery will inspire the same sense of outrage the "traditional" one will in most people.