Before the coffee, the equilibrium was for the line to be 100 persons long.
After the introduction of the coffee, the equilibrium changes so that people are willing to wait in a longer line than 100 people, although the individuals themselves don't have any preferences (like A only standing in line if it's less than 20 people and so on).
I think not much thought was given to explain the answers in depth since the main point was to show where GPTs reasoning was lacking.