Nobody was trying to equate non-disparagement clauses with slavery. The relevance of slavery here is as an example of the kind of contract terms that everyone should be able to agree are rightly invalid and unenforceable. Any argument in favor of contract enforceability that would apply to a slavery contract just as easily as it applies to a non-disparagement contract is a bad argument, or at least woefully incomplete. Bringing up slavery serves as a necessary reminder that the details and nuance of the contract terms and their effects need to be discussed and argued, and that an unqualified "contracts should be valid" position is untenable and oversimplified.

The general principle is that you shouldn't be able to "sign away" something that's a constitutional or human right. Like the right to freely speak, the right to practice a religion, the right to be paid for work, and so on. Imagine if the severance contract specified that she had to convert to Islam in order to get her severance, or that she had to sacrifice a child. No court in the country would consider those clauses conscionable. Yet, somehow companies are allowed to gag your free speech as a condition in a contract? It makes no sense why this is allowed.

This is legalized buying people off, yes these contracts ought to be illegal and the comparison to slavery (a worse, but same category of morally reprehensible power dynamic) is completely valid