This is what was actually stated:
While holding at flight level 220, the flight crew received a stick shaker activation and detected airframe buffeting.
When did it turn into an encounter with severe turbulence? Lets rewrite Derfred's statement of fact with the real facts and take a different point of view.
1. The stick shaker activated and the aircraft encountered airframe buffeting while in the hold at FL220.
2. The autopilott was disconnected and the flight crew maneuvered the aircraft in response.
3. Passengers were injured.
Now for the conjecture. The hold speed for a 747 is approx. 230-240kts at a rough guess. That's not slow! If you were coming into land at that speed it would be frighteningly fast. The stick shaker activates approx.10kts before the stall which would be possibly around the 120-130kt range. What was the A/T doing during this time? I would hazard a guess that it was not armed but the crew thought it was, not unusual in a Boeing if recent history is anything to go by. It would not take long for a 747 in level flight to have its speed decrease to the point of stick shaker activation. Something was distracting the crew so the speed was not being monitored. Why the speed wasn't being monitored and why the A/T didn't maintain the holding speed will come out in due course.