Apparently, even the Austrian school is teaching knee driving with upper body rotation.
The only way I can answer this is that the best Austrian racers don't knee drive and don't rotate. But even in some of their best ski instructor skiers, there is rotation and knee drive. So the only conclusion you can draw from this is, the Richie Berger types, figure out how to ski correctly. Even if they don't know how to explain it. I have heard Richie Berger explaining skiing in German, in an interview, he sounds no better than a PSIA instructor doing the same. However he can ski. So the two are disconnected. The more you work with some top athletes , not all, the more you find out their skiing and their knowledge don't always go hand in hand. There are 3 or 4 on the Austrian squad that are very good skiers, however they may not have the correct understanding. Don't forget this is ski school, not Austrian racing school coaching that the Austrian kids, like a Fenninger, get at Stams, the Austrian nation racing academy.