I read that description earlier today, too. However, considering how new the system is, I was wondering if those truck(lorry) drivers were trying to open an exit that was designed to stay closed in just such a situation?

This is completely conjecture on my part, but in a tunnel fire, there is going to be massive amounts of smoke and the only places with air will be inside the train or in the adjacent service tunnel that passengers can escape to. If the emergency procedure is to wait for train personnel to direct you to the best exit, then smashing out windows wherever you are could just be letting smoke into the train unnecessarily.

Again, just total speculation on my part. I'd love to hear any comments from the authorities regarding these "jammed emergency exits".