I don't know how accurate the stories are. The misinformation that went to Europe in those days was famous. But Montaigne is clearly impressed by the innate morality of the tribes. He is especially taken with their bravery and tells stories of men who would rather suffer death than show fear.
The cannibalism that he speaks of is done after a battle. The dead are eaten as a way of showing extreme revenge. He says that this treatment is favorable to the various methods of torture then employed by the supposedly more civilized Europeans.
Near the end of the essay Montaigne talks about a handful of the natives who were brought back to France. He makes a prediction:
Three of these people, not foreseeing how dear their knowledge of the corruptions of this part of the world will one day cost their happiness and repose, and that the effect of this commerce will be their ruin...This is more or less what has happened. Many native tribes have struggled to find their footing in western ways, even now, hundreds of years later.