The essence is that a norm would become established that such an ambition would just be insane. He makes a comparison with parliamentary democracy, where losing parties peacefully relinquish power, because nobody doubts that that is what they are supposed to do.
I think his argument is invalid, and that is based on a fundamental difference between the positions of the outvoted democrat and the security company.
The difference is that the democrat has a binary choice. He either accepts the result of the election, publicly, in which case he must step down, or he rejects it, in which case he is making it clear he is breaching the established expectations.
Private security companies in the Rothbardian sense are not forced into the same “in or out” dilemma. If one wants to protect a piece of property for one client while another wants to protect the same piece of property for another, there are infinite gradations of conflict they can resort to. The lower levels are deniable (“accidents” for example), middling levels can be justified as peaceful bargaining — “sanctions” of various kinds, and the highest levels of conflict, while still consistent with a dispute over a particular legal question, are indistinguishable from a war of conquest. The ability to vary the level of conflict in small steps allows the kind of norms that apply to democracy to be eroded or made irrelevant.