Before jumping to conclusions, let's look at the numbers:
The duration was about 10 seconds, the altitude excursions was 500 ft.
There is no way we can derive a 15g acceleration from that.
It could have been, for example, a 3g acceleration over 2 seconds, resulting in 200 feet excursion, followed by 0.2 seconds coasting after which the pilots levelled off with 2.3g in 2.6 seconds. The resulting excursion would be 500 feet. If the processes then repeated in the other direction, the total time would be approximately 10 seconds, the maximum excursion 500 feet and the maximum acceleration only 3 $g$.
But the fact is we don't know, it's pure speculation.
Another point of discussion is the vertical travel. How was that measured? Very likely it was measured by the altimeter, which works by measuring the static port pressure. How do we know that the static port was measuring static pressure? Very likely it was not, because the turbulence would probably have had a lateral component, as indicated by the 20 degrees left and right rolls. Lateral wind (sideslip) means the static port is exposed to the airstream, and therefore measuring a mix of dynamic and static pressure. It could have been more than 500 ft, it could have been less. We simply don't know.
The most reliable way of determining g forces is by measuring them on-board and recording them (e.g. in the flight data recorder). With the data we have, there is simply no way to know what the g-force were.
In my opinion, it's very unlikely that the aircraft would have encountered 15 g.
The fact that an aircraft is certified for 2.5 g, doesn't mean it can't withstand more, as the crew of China Airlines flight 006 demonstrated on 19 February 1985. They managed to reach 5 g and bend the wing permanently 5 cm upwards. The aircraft was repaired and flew for another 20 years.
Note that the 2.5 g is for load due to manoeuvring, not only for turbulence. The certification specification for large aircraft on the subject of turbulence and gusts has changed several times since the certification of the DC-8.
In 1964, a formula was introduced describing the gust load that the structure aircraft has to deal with. This was added as FAR 25.341. This section has subsequently been updated in 1990, 1996 and 2015.
In addition to changes in structural requirements on the airframe, the origin of mountain wave turbulence in much better understood nowadays. It is taken into account in the operation of the flight.