If he had such expertise he should have sent his reply with all the recipients on bcc. Then even if a few people do reply they only go to him and things fizzle quite quickly.
I don’t quite why so few people break reply storms in this way. Bcc isn’t _that_ mysterious.
The reason is that a bcc'd message doesn't stop people from replying to earlier messages in the thread, whether or not they are using a threading email client. Threaded email reading interfaces probably make your strategy more effective, but they don't eliminate the possibility that people will respond to earlier messages, defeating your "firebreak" message (the more addressees, the more likely it is).
It's probably because he felt that he was in a position of expertise and authority that he tried to pre-empt the problem by telling everyone not to reply to the list. One last message to all, then the problem would be definitively solved, or so the logic would go.