There is a concept called armor density.
Western tank designers often trumpet how spacious the insides of their tanks are. That's lovely, but comes at the expense of giving you a far larger area to protect. Meaning you are going to be significantly heavier to achieve the same armor thickness. Lighter does not automatically mean less armored.
Besides, the designer is probably not comparing the 'naked' Type99 to western tanks, and would have likely factored in the ERA and laser dazzler etc as well.
The most dense common element that is relatively stable is uranium, and unless the Chinese happened to build their tanks out of uranium, not possible.
Western tank designers place a lot of emphasis on
crew fightability. The crew need to be comfortable in their tanks because they spend hours in their vehicles. Otherwise, crew mental and physical exhaustion and fatigue will build up rapidly leading to less effective crews in their tanks. A tired, fatigued crew is not combat effective, especially in extended combat.
Furthermore, the West have placed a lot of emphasis on ease of operation. Visibility is excellent out of most Western tanks, and the three person turret allows for more eyes on the battlefield, while still allowing people to operate radios, weapons, or commanding. The commander of the tank, for example, in a three-man turret, can concentrate on commanding, while in a two man turret, that commander will have to manage a task on top of commanding a tank, and that will distract the commander, and prevent him from operating the tank effectively.