Deterrent buffet, test in A340 vs A330
While I'd agree that the A340 and A330 are - intentionally - very similar, this buffet appears to shake the cockpit much more than the cabin, yet is presumably excited by the air over the wings as the flow starts to change, rather than by the air over the forward fuselage.
As a result, the number and shape of the engines, affecting the airflow around the wing near the engines and the elastic and inertial properties of the wings, might be a relevant thing to look at. The weight distribution in the forward fuselage might matter too for how the cockpit wags around.
Presumably the test pilots were confident that the A340 results would apply to the A330, and convinced the investigators and the magistrates. Perhaps at 1-10Hz the differences are small - that certainly seems to be more of a shake than a bend.
If it's their own test A340, then it might also be safer to do this extreme test on board a very familiar aircraft.