This was cool, but I have no idea of what was achieved or what the significance of it was. Other than it was possible to receive a signal during heavy rain.
The author pointed a telescope (his directional ground station antenna) straight up and on a rainy day and could see the light bulb turn on and off.
This is also how "troposcatter" shots work with microwave radios. If the distance between two stations is too far for line-of-sight (due to earth curvature effectively being a "hill" in the way), you can bounce the signal off of the troposphere.
That being said, wet trees are more effective at attenuating non line of sight radio signals than dry ones, even at 2.4Ghz and below. So while the raindrops themselves have minimal effect for sub-10GHz signals, you can see signal fade in wet conditions.