Nostromo's "victory" over Monygham is the victory of the dream-ideal over cynicism, and the victory of the People over the Devil. Note that "fidelity, rectitude and courage" are the attributes Monygham lost during his disgrace in prison. Nostromo has "the assumption of" those virtues, and the cynic is eager to uncover the reality of vice in him, to learn that he too has lost his integrity. We know, and Monygham guesses, that in fact he has -- this is the Devil trying to reveal the cynical truth about the People. But he is stopped cold by the stubbornly persisting dream-ideal of integrity. Note that Monygham is defeated by the "magnificent Capataz," the idealized faithful People.