Well, here is my problem with this. I can tell your for a fact (I'm running it right now) that installing the GTX2xx optimized drivers for any of the 8xxx and 9xxx cards gives about 10% - 15% boost in performance. Since they are using 177.39 drivers which are effectively the official GTX2xx-type drivers packaged for 8/9xxx series, I can say pretty comfortably that is where they are getting the performance increase from. Having said that, I again have some issues with this review...
(1) When they are testing the OC on the HD4850 they use Catalyst to OC it, and they do not even get past 690MHz there resulting in a 65MHz boost. On the other hand, they OC the GTX+ with EVGA proprietary tool to 102MHz more than the base core speed resulting in a 840MHz core. Therefore, the comparisons are not exactly fair.
(2) They are testing NVIDIA's optimized driver against the first release of Catalyst driver for the 4850, which again does not really hold up.
(3) Last strange thing I saw is that GPUZ seems to identify their GTX+ as a regular GTX, at 65nm process. I'll assume GPUZ is wrong, or else this whole review is garbage.
In all, even if this review is true I'd say that you should expect the HD4850 to be able to pull ahead when, (a) the drivers mature a bit, (b) when manufacturers do some aftermarket cooling on these cards, (c) when the aftermarket OC tools start to support the HD4850 so you can comparatively OC the two (this I bet will be the biggest difference maker), and (d) when the price of the HD4850 drops by another $20-$30.