I spent a month in Cuba in 2008 and though I didn't see any signs telling people to honk when overtaking, everyone was doing it on the highway.

A lot of cars in Cuba are in terrible shape, but it might also have to do with the fact that the Autopista 1 is in very bad condition. You really have to focus on avoiding the massive potholes, so you have less time to pay attention to your surroundings. Honking helps the driver in front to make sure to swerve away from you while you're overtaking on the next pothole.