Most of them were not surprised. Brazilian jiu-jitsu, a form of modern martial arts, combat sports, and self-defense techniques, has taken over the world. It is one of the most effective and lethal forms of martial arts one can possess, which is why it effectively saved the man in this story from a much larger opponent.
This goes to show that outweighing and outmuscling an opponent does not mean much when he/she has some jiu-jitsu skills. The smaller man was able to take his larger opponent down so quickly that the larger man must have been left wondering what in the world had happened.