Of course part of Type 99 might be derived from T-72, like the V-shaped thing of the hull you pointed out...but first of all the entire hull is not derived from the T-72...to make the hull one meter longer and change the placement of the engine, there must be many many innovations made to the internal of the hull...
the only major feature in common between T-72 and Type 99 is that the hull looks a like on the outside...but having few things in common doesn't mean they're derivatives of eachother...for example, the Abram has a Leopard's main gun, but that doesn't mean Abram is based on Leopard; india's Arjun's turret looks like Leopard's turret, but that doesn't make Arjun Leopard's derivative; the Japanese Type 90 also looks like Leopard, but that still doesn't make Type 90 Leopard's derivative...oh wait, type 99 has add-on armour similar to the Leopard's style, that must mean that type 99 must be based on the Leopard? of course not...you gotta draw the line somewhere.
having one feature in common doesn't mean the entire tank, type 99, is based on T-72. The external looks of the hull is nothing compared to the rest of the features of type 99. When it comes to the tank, Type 99, it is an indigenous design...