In reading through the article, what I see are illegal aliens who enter into employment willingly, and suffer harsh working conditions and terrible pay. In other words, they are exploited.
The article comes right out and says that "Raids can end up sending migrants without proper paperwork to jail, while [shrimp factory] owners go unpunished."
There's that word. "Migrants". In other words, people who don't belong there in the first place are being exploited for labor. Doesn't make them "slaves".