It’s not a wise decision to hit someone’s car to get their attention.

I assume he was on a bicycle in the earlier incident as well, not in a car.

With a car, sure. You tap your horn. You don’t bump the car that is oblivious to the light change.

But the guy probably doesn’t have a horn on his bike, isn’t required to have one, and tapping the bumper with your hand is not going to hurt the car in question.

There are things that bicyclists do that very much annoy me, like blowing through red lights, riding the wrong way on the road, and riding with no lights at night. Tapping my bumper isn’t one.