His uncle is, indeed a good person. But the whole "if you're strong, you have to protect the weak" sounds like "with great power comes great responsibility", which is a fallacy.
Martial arts, just like marksmanship, archery, and any hobby that could be used for defending a person in need, don't come with an oath. People do them because they like them. People indulging these hobbies may be pacifists, shy, against violence, or just cowards.
To be precise, I think the uncle is wrong in scolding him for not fighting the bullies. He wouldn't be wrong if he scolded him for not doing anything. The bullied kids would be in for a world of retaliation if they complained to the teachers, but Kang Yeon could do it in their stead and still be able to fight the bullies in self-defence if they came for his neck.
Passive observers as bully enablers is certainly a view I share, though I've come across people who disagree when I was reading Revenge classroom. But I disagree that being stronger means your responsibility is necessarily greater, at least in the way our society is organised. It might be argued, though, that it is because strength means he has more tools at his disposal that ordinary people, but one doesn't follow from the other. A weak person could have recorded evidence of the bullying from a distance. A girl, who might be protected from physical violence (unless it's the gangster-like bully we see in some series), could be in a position to denounce the bullies, too. It doesn't follow that only people who can fight with their fists are supposed to take responsibility for that.
But I understand why the uncle thinks so, given that's the way of life he's professed. But it's not necessarily right or the only way.
I hope he meets his uncle again.