A monopoly is not a "state-imposed single seller". Where'd you get that definition from?

There are many examples of natural monopolies, which is a concept that has been recognized in economic theory for at least two centuries. Walter is either being deliberately obtuse or is ignorant of history.

AT&T, Western Union, most railroads, many local utilities—all examples of monopolies that formed "naturally" i.e. without state intervention.

Insofar as there was state intervention in the formation of these monopolies, it was because the state intervened on the company's behalf to suspend antitrust law—such as in the case of AT&T, where the government decided it would be most efficient to permit AT&T to continue operating as a monopoly (until it didn't, and broke AT&T up).