The "sly simplicity" is Conrad's, for this verdict has a hidden symbolic
meaning unintended by Dr. Monygham. The "pretty face" that Nostromo
cannot refuse belongs to Paquita, who represents the rulers, while his "gambling"
(his game, we recall, is monte -- read mountain, or ideals) represents
his constant service to the great ideals of history in the vain hope of reward.
The doctor's verdict thus translates roughly to: "The People are poor because
they do everything the rulers tell them to, in a stacked game that they can never
win."