Not to mention that Type 99A showing up in high altitudes doesn't mean it's above 4000m where the confrontation sites are. Maybe it's just as useless there. It also doesn't mean it'll be as mobile as Type 15 even if it could potentially operate in high altitudes. That means they are very different tools even if they are fighting alongside one another which I honestly think is unlikely for extreme altitude. Type 99A and other PLA MBT all pass Tibetan plateau training but those altitudes are far more manageable than Ladakh.
The Type 99A may prove to be less than ideal for warfare at Ladakh. But I think it is still wise for China to experiment with the tank at such an extreme environment. There are many new things that the researchers could learn when the tank is exposed to it.
Still, the Tibetan Plateau is already an extremely punishing environment in and itself. Even better that live fire exercises have been carried out there. Very important things would have been achieved, such as gun ballistics calibration for high-altitude warfare. Maybe also special camo paint for the higher UV levels. If the designers could find more ways to improve the performance of the tank in an even more extreme environment than this, its always better for the tank.
The Type 99A would find a much more crucial role, fighting on the steppes of the Tibetan Plateau. Rather than in the mountains of Ladakh. Now that the tank is being optimized further for high altitude warfare, its all the better for the defense of Tibet.