This effect is very much not limited to monopolies, though it's certainly easiest to see there. There's no step change from monopoly to competitive marketplace though. If you believe it's the company's moral duty to provide e.g. healthcare then in a non-monopoly situation that culpability is divided, though not abrogated (and beware the bystander effect!). From the employee's perspective, the spectre of physical harm is a bit further off, but it will still colour negotiations.

It's especially insufficient to generalize the working of the entire system from an example of a market in which employees currently have enough power to not really have to worry about the prospect of physical harm because it would be disadvantageous to the employers to cause it. Even if we take the current state of the SaaS startup market as reliable (which it isn't) the original argument was not limited to SaaS startup employees, and in other industries (including ones that are a bit down the pyramid from the SaaS companies) things are a lot less rosy for employees.

A sole consumer of labor is a monopsony, not a monopoly (that would be a union). At any rate, the point is that there are many many employment negotiations that no reasonable person would agree to amount to duress. This is a counterexample to the idea that any negotiation of employment involves duress. I don't need to disprove the existence of any coercive employment. But SaaS companies are especially relevant since pg specializes in showing people how to become billionaires through SaaS. If earning a billion dollars implies some measure of coercion we should be able to find that in a SaaS startup.