The premise is that as long as the monopoly business is not engaging in unethical business practices that it doesn't matter if they're a monopoly. All ancap ideology focuses on is barring the unjustified use of force or fraud in interpersonal relations. As long as a big company doesn't break those rules, then they can only maintain monopoly status by doing a fantastic job such that nobody sees an opportunity to compete with them or is able to successfully compete with them. That's perfectly ok in ancap world.