What is the significance of the title of chapter 7, "A Pepsi for the Apocolypse"?

The title is significant in that it highlights how vulnerable the migrants were. Also, the title reinforces how unprepared the walkers were to deal with desert conditions.
In essence, the words "A Pepsi for the Apocalypse" demonstrate the futility of relying on inadequate tools to triumph over a substantial challenge.
When Maradona failed to materialize, Mendez became the lead pollero. However, he provided the migrants poor advice regarding what provisions to bring on the journey. The walkers bought Pepsi, chocolates, salted prunes, and other snack-type food before they set out. Many did not purchase adequate water for their journey. This was their first mistake.
Perhaps, their greatest mistake was in trusting Mendez, who soon showed that he was far out of his depth in his leadership position. For his part, Mendez ordered the bus driver to drop the group off at El Papalote, a desolate area. At this point, the United States border was only less than a hundred yards away.
Despite this, Mendez soon piled the men into a van and drove to another location. The ride was an uncomfortable one and later, surviving members of the group testified that it took at least 2-4 hours to reach the "big rock."
In all, the migrants believed that they would easily cross the border into the United States. So, they made few preparations for the monumental challenge before them. The reference to Pepsi demonstrates their lack of preparedness and the futility of relying on inexperienced polleros. Instead of finding freedom, the migrants faced an apocalypse of immense proportions. The word "apocalypse" refers to a tragedy or catastrophe of great scale. The men found themselves facing an unforgiving terrain and intractable climatic conditions, immense challenges they could not overcome. Their unpreparedness and Mendez's negligence resulted in a massive loss of lives.