One has to wonder. ETOPS divert is at 320-335 KIAS. It will be interesting to see if the A330 crew maintained that speed or if they reduced speed to reduce the vibration.
Changing the speed in either direction, up or down, likely would reduce the vibration in the cabin.
Of course it's up to the pilot to set the speed according to his needs.
As stated earlier, the issue is not related to structural integrity. The pilot simply follows his training.
We'll see what the ATSB has to say about this eventually since there is always room for "lessons learned"