Actually, I've found that my first reaction in any traffic situation is to get out of it, either by maneuvering or braking. I never seem to find time to honk (or even think about it) and I'm puzzled as to how others reach for it the first thing.
I do both simultaneously. Both have saved me. There have been times where I immediately hit the brakes and horn when I saw someone doing something stupid (about to pull out into traffic, about to merge into me from another lane, etc) In some of these situations, the horn causes the other person to stop what they're doing and a collision that could not have been avoided solely by me braking, is avoided.
I have seen this with a lot of other drivers. My very first reaction is to brake. However, in all of my close calls, I have never noticed the other vehicle even attempt to slow down- they just honk their horn and continue on their way.
You're right, but if you watch videos of accidents, or talk to people that have been in, or seen a lot of traffic accidents, you'll find most people first honk, then think about braking.
I personally think it's because people have the "get out of my way" mentality, and don't want to be inconvenienced or slowed down when they know they're in the right.
He honked only after the car braked, so at that point there was no reason to focus on braking.