You may be right. From other reports: "Eurotunnel's Gounon said people suffered smoke inhalation injuries because they evacuated the train themselves before tunnel operators had ventilated away the smoke, as safety procedures call for."

And: "Gounon said safety precautions had worked "perfectly," and he dismissed the matter of truck drivers smashing windows as a "normal" reaction by men who "perhaps wanted to get out faster than necessary from the pressurised cabin."

He told French radio the automatic doors on the shuttle only function when it is certain that safety fans are working so there is no risk of anyone being overcome by fumes."

Although I doubt anyone EDCs a hammer, so presumably it was provided in the train specifically for the purpose of breaking windows in the event that the emergency exit failed. (Unless it wasn't actually a hammer but a multitool or something carried by the passengers.)

There is another report of other passengers forcing a door open. Maybe it just opened on its own after the fans started.