The root error of this article is the mistaken thought as if there exists such an animal called "employment-contract".

No: rather, there are contracts. Period. Whether or not people choose to classify certain contracts as an "employment-contract" is similar to whether or not they classify the platypus as a mammal: the platypus is there, that is a fact, but its classification as "mammal" is only a title of convenience.

A contract could include a clause saying "X will work for Y [doing this-or-that, at such-and-such times]", or a clause saying "Y will give X a salary", yet some contracts could include just one and not the other, or perhaps include alternate, similar or weaker clauses. The requirement for X to not harm the commercial interests of Y and/or to not speak against Y (in private and/or in public), is optional and is not automatically included in every contract.

Another basic error, is the claim that "A person also cannot contract to become a slave".

False: anyone can contract to become a slave, despite the fact that the state and its government would never enforce such a contract. A contract is one thing and its enforcement is completely another.

The "need" to TRY to limit what people may include in their contracts, arises from the cruel reality (mentioned by Armchair Critic) that "People need jobs to survive". Modern society has created this cruel reality and so it is modern-society's responsibility to provide the remedy - rather than to throw that burden on the shoulders of individuals which it arbitrarily classifies as "employers".

An obvious remedy would be the provision of UBI (Universal Basic Income), so that nobody's survival would depend on having a "job". Comforts and luxuries yes, but not survival!

Once a remedy is provided and having a "job" is no longer required for survival, nobody will be forced into unwanted contracts under duress, including such that involve physical, emotional, mental or intellectual slavery. If one still does enter such a contract despite their not needing it for their survival, then nobody else is to be blamed.