How is unfree labour as characterised by Lebaron and Ayers (2013) distinct from "modern slavery" used by international non-profits and intergovernmental agencies to describe exploitative practices prevalent in cases such as Africa's incorporation into global supply chains?

Reference: https://www.researchgate.net/publication/263375757_The_Rise_of_a_'New_Slavery'_Understanding_African_unfree_labour_through_neoliberalism