The flask was far from full and it doesn't help that its top is tapered. For an effective demonstration of what you've just described, there's a video of a German Leopard 2 doing the same thing but with a full mug of beer and with the tank driving over bumps.
The Leopard had a special flat platform mounted to its cannon and the beer was only "full" because it was a third foam on top. The foam actually acts as a cushion against smaller violent turbulence in the beer. The cannon also did not swivel.
There is also a Japanese video and they too, required a flat platform and the tank barely moved on flat cement. They used red wine, filling the glass less full than the Chinese water jar, and it jarred much more violently than the water in the Chinese jar, but the glass was perfectly stable because of the flat platform, highlighting how much easier it is on a generous flat surface.
This was clearly an on-the-fly demo with no modifications to the 099 so they just wedged the jar in on a round turret. The Chinese are the only ones to attempt this without any flat platform and without any sort of foam to keep the liquid from jarring. And quite frankly, they filled it more fully with liquid than either the Germans or the Japanese. Furthermore, the jar moved slightly side-to-side since the piece was never designed to hold it, so it was far from secure. The only thing that made it easier was the tapering of the jar but to be honest, the water was so restful in it that if it were not tapered but at the same height, it would still have prevented the water from spilling.
The omission of a platform and the calmness of the water without any foam protection make the Chinese demo clearly the most difficult.