I agree. I too believe that this lack of side hull armour on the Type-99 tanks is a deliberate decision. These tanks are expected to be fighting in traditional armoured formations. Where flank protection can be covered by wingman tanks and other units. These tanks are not intended to fight in urban battlefields like Grozny or Fallujah. Since the PLA's doctrine is mainly defensive, there is little need to play the occupying force where urban combat becomes much more common.
Anyway, additional side hull armour is heavy and bulky, thus impacting mobility. If they are not needed, why have them bog down your tanks. We can also see this logic with the other top tier tanks in East Asia. The Type 10s and the K2s. These tanks too generally lack side hull armour in their current configuration. Even the Abrams did not have that impressive side hull armour until they experienced Iraq in 2003 onwards. After that, the Abrams begins to get the TUSK package. Hence, I believe the manufacturer could quite easily have a 'TUSK' package for the Type 99s on the catalogue. An add-on package just in case they these tanks do need to go into urban combat.
Some of the loudest voices that I have heard bashing the Type 99 for its lack of hull side armour are from the War Thunder players. They think that by driving a tank in a video game, they have suddenly become tank experts. This is laughable. For example, a tanker in War Thunder could target and snipe out individual 'weak points' of an enemy tank. But a tanker in real life could only make out an enemy tank's profile on his FLIR at 1800m out. He could hit the enemy tank, but he won't be able to pick out which part of the tank he wants to hit. Not to mention the other 'game balancing' things that distorts the realism even more. Hence, War Thunder is not real life, and we should not take it too seriously.