I don't liken it to slavery, but most wage (or salaried) labor looks like this. Making sure your employer thinks well of you (your 'rating'), getting certifications (some employers will pay, but not all).
A gig employer is unlikely to work with you again regardless. They don’t have much of a reason to care about you. And if they rate you poorly, that can potentially shut you out of the market entirely. Or cause you to pay for costly and timely retrainings like the article.
It’s a much harsher employment contract imo.