The Evo X is heavier than the Evo IX, and the new STI is a bit more powerful and a bit lighter than the Evo X.
Edit: I forgot to include that the new STI has a close-ratio 6-speed, while the "regular" Evo X has a 5-speed. This was probably the biggest factor in the STI's victory.
Also, why the twin-clutch SST version of the Evo X did so poorly is beyond me -- either it sucks or the driver wasn't as good as the others. The time attack results, as seen at the end of the video, seem to indicate that the SST is as fast as the outgoing STI, and a bit faster than the 5-speed Evo X.