the practice, which is tantamount to modern slavery
Surely the practise is just traditional slavery slavery.
Kidnapping people and imprisoning them with threats to do work for you is the very definition of enslaving them.
The cynic in me rather suspects that the term modern slavery has at least in part been invented so NGOs can say, "well aktuwly there's more slavery than there's ever been". Where they compare modern slavery and slavery from now against just slavery from back then and ignore the modern slavery things that also happened in the past. Because apparently it's impossible for a modern NGO to say something bad is happening, we must try to stop it. Nowadays everything is either a something hypen "emergency", "crisis" or it's "the worst that it's ever been in history".
On the other hand, we'd rightly call apprenticeships from 200 years ago modern slavery. A job where you have to live in a specified place, can't quit and can be forcibly returned if you run away - where you must work for 7 years to earn your freedom. On reduced wages (though you did get board and lodging) - plus you aren't even allowed to marry without the master's permission. Back then it was actually quite a good deal, you got training that was otherwise unavailable and the chance to join the master's business and possibly even marry his daughter (if there was one). So you might legitimately argue that it wasn't slavery then, but is now.