Factual error: Most of the time the boys have no visible breaths, even though they're in a freezing climate.
Factual error: While debating when to begin the trek across the Andes with Nando, Roberto remarks that "winter will be over" soon. This incident took place in South America where the seasons are opposite from the northern hemisphere. The plane crashed in the spring and Nando and Roberto reached Chile by foot in the summer.
Answer: Keeping in mind that some parts of the audience were not familiar with the details of this story, using Paez as the narrator retains some of the suspense of Canessa and Parredo's journey across the Andes. If the audience is not told at the start that they survive, at least some viewers may have considered that one of them may die during their trek to civilization.