I think a few people are letting their imaginations run away with them. I'm no great supporter of extinguishers at all but I recognise that they can help to protect means of escape, but only in the following way and not in the way described by others above - quite simply, if a fire breaks out in a building and is extinguished by someone with an extinguisher then the means of escape for everyone else in the building has been protected.
They deal with the root problem and if they don't work then we have more recognisable elements to protect escape routes.
There is no question that their purpose might be to protect the extinguisher operator's means of escape, we all know that is nonsense; he or she will have other fire safety features protecting his or her escape route. The extinguishers, if used successfully, will protect everyone else's means of escape.
The FSO, if quoted correctly, didn't quite get it right. What he might have meant was, "one of the benefits of extinguishers is that they might contribute towards the overall protection to the means of escape."
Stu