It threatens physical aggression against people for engaging in certain kinds of voluntary exchange. To me, that is coercion. Just imagine if your neighbor decided that he would impose a minimum wage law on us. Wouldn’t we all agree that he was coercing us? If it is coercion when he does it, why isn’t it coercion when the government does it?
While I have some sympathy for his general project, Klein's essay risks reinforcing three conceptual errors of libertarian ideology:
(1) It neglects the coercion inherent in the very institution of property. To claim ownership of a resource is to prevent others from making free use of it. If another attempts to use the resource in the same way as you do, you can call it "theft" and initiate force against them (or have the police do so on your behalf).
That's not to say that property ownership is necessarily wrong, of course. But you can't pretend that laissez faire provides any sort of neutral starting point. It involves coercion, just like every other system. The real question, then, is how significant are the impediments created by each institutional framework, and whether the opportunities they open up are worth it.
(2) It neglects other kinds of constraints that can impede us, leading to an impoverished conception of "freedom" that fails to track what really matters to us (namely, capability). Negative liberty is fine as far as it goes, but it makes for a rather one-eyed approach to evaluating policy. A better maxim would be to seek to enable people to achieve their goals. Economists (like everyone else) should be concerned with opportunities, not merely interference.
(3) It conflates personal and institutional action. This is the difference between vigilantes and magistrates. Just because it would be illegitimate for your neighbour to do something in their role as an ordinary citizen, doesn't necessarily mean there's no legitimate way it could be done.
A well-ordered society is governed by the rule of law. This means that there are institutional processes to govern certain classes of action. The outcome of a just institutional process -- whether it be a guilty verdict, or minimum wage legislation -- has a different normative status than the corresponding action of a neighbour who takes it upon himself to unilaterally impose his will on others.
The upshot: yes, instituting a minimum wage involves an element of coercion. But not in the same way as if your neighbour did it. More in the way that instituting property itself involves an element of coercion. Whether either set of laws counts as "coercive in any significant sense" will depend on context. It's not as cut-and-dried as someone who makes the above three errors might assume.
Finally, I should emphasize that the alternative to ideological libertarianism is not a blank cheque for statism. I wouldn't claim that "whatever stuff you have really belongs to the government", or anything like that. It's possible to set up an unjust institutional order, and even within a largely just order it's possible for government agents to violate their (e.g. constitutional) obligations. So there's plenty of room for political criticism. The point is simply to warn against the complacent assumption of laissez-faire as the "natural" or default system, to be contrasted with all the "coercive" alternatives. In principle, it ain't so different. As with every other system, it must be assessed on its practical merits.