Maybe a made a typo or the article had a typo on the extension thing - logically if some one needs an extension, they would probably not be able to comfortably sit with the armrest down.
Other question - how much (if any) is it a safety issue if a person doesn't fit adequately in the seat? If they spill in your seat does this constitute a safety issue for you? All I know is the announcement says on take off and landing that the armrest should be down - if I have that correct...- if so this could be dangerous for the passenger forced to sit in the seat where an armrest has to be up to accomodate the larger person.
Planes have crashed because of overload, or because of bad payload distribution. Endangering an entire plane, passengers, crew, persons on the ground by going with antiquated estimates of what one passenger weighs makes no sense - and the same problem has come up with elevators, where average weight per person used to be way under 200lbs.
Engineer said it best, passengers and their bags should be weighed together and count as one payload. On the seatbelt extension - you made no typo, I didn't clarify sufficiently.