I don't see the contradiction. They didn't exterminate people of German descent in the conquered territories, rather they were supposed to become the new elite.

Focusing on exterminating of social elite (doctors, lawyers, famous artists, officers, writers, journalists...) is admitting there was an elite. How can a slave race have people creating culture and order?

You cant challenge Nazi racial ideology as if it was a logically consistent system. It was pretty opportunistic. For example Slavs are technically Aryan (given the theory behind the concept of an Aryan race), but nevertheless the Nazis considered them subhumans because it justified the lebensraum politics. On the other hand, the Japanese was elevated to "honorable Aryans" when they became allies.