If I’d encounter someone wielding a naked 15cm blade, I’d be extremely wary as well. That blade can easily cause serious harm or kill, regardless of being a replica toy or not. Walking around with what is essentially a dagger is reckless. Maybe the sentence is a bit harsh, but that dude deserved being stopped.
Yes, you're right everything can cause serious harm or kill, but certain things are designed to harm or kill, or designed to look like something that could harm or kill.
Context is a big factor here. A person holding a 6 inch knife in a kitchen? No one is going to bat an eye. They're probably using that knife to prep vegetables. Same knife, same person but they are walking around a market/playground/movie theatre? Suddenly it's a very different situation and context.
Is that stranger with the knife safe? I don't know, but I do know that I don't have anything to defend myself aside from what is around me if they ARE NOT a safe person.