There was a previous interview with the Devs and when asked why they used the CryTek™ Engine. They replied that they "adapted" the engine for use with Aion because of its ability to render lush environments. The "key word" being -"adapted"-. This is the same issue that my fellow co-workers and I argue about day in and day out. If one goes back and looks at the history of DX9 and then analyze Aion's enviromental vids. You can clearly see that there are a LOT of environmental variables at play here. More than what DX9 is capable of handling based on current hardware specifications.
Perfect example of such environmental rendering is the real quick glimpse of a stormy sky above one of the charcters. Look at the flouidity and the realism associated with that clip. Nothing I have ever seen using DX9 looks like that. DX10 allows for such computations and also allows for more immersive environments in that it allows the GPU to render MORE unique objects within a single frame.
Oblviously they can't make the game DX10 only... but if it is DX9 only.... then NCSoft has some crazy uber programmers to pull off such dynamic renderings while only utilizing DX9.