Average after several years of compiling data may be 77kg. That is the nature of statistics, it tends closer to the average the more data you collect.
That doesn't mean in any way that a plane load of overweight tourists from the USA will average 77kgs.
Or that any particular flight on any particular day may not be overweight. Certain sectors eg peak MEL-SYD weekday flights most business travelers bring carryon and don't check their luggage. The carryon is certainly not within weight limits (rarely weighed and enforced). Multiply an extra 5-10kg per pax by 250.
I remember once carrying a large bunch of Japanese Sumo wrestlers. They had to block out adjacent seats just so they could sit down. But they were boarded using standard weights. And those standard weights were used for takeoff performance data. There should have been an extra couple of tone in that for sure.
The reason standard weights are permitted is that there is enough fat in the performance limits that a small overweight which WILL occur from time to time using standard weights will not have any material impact. Even WITH an engine failure.
The article is correct about internal systems being inadequate, but not correct in its suggestion that this particular incident possibly posed a danger to flight safety. I would suggest that you would have to go at least 10-20 tonne over before you would really start to worry about flight safety.
Having said that, if they hope to ensure a fix to internal systems to prevent future recurrences of possibly larger magnitudes, they probably feel the need to play the safety card, and I don't have a problem with that at all.