Apple has had an agreement with Google since the announcement of the iPhone. Google's CEO Eric Schmidt even sits at Apples board meetings.
Google is a special partner and this is why you get Google Maps integration, Google as the default search, etc... It only makes sense that they get preferred treatment in other situations as well.
Actually it looks like Google was just using a private, undocumented API. Apple likely did not give them special access. The blogger was able to gain access to the method later.
From using the app, it looks like they just use the accelerometer and guesswork. Or they just really suck at detecting my head. The preference for screen rotation says "Turning on Screen Rotation disables motion sensing for voice search."
The article has been updated to reflect that it's overriding an undocumented method on the UIApplication class called -proximityStateChanged. I've tried it and it looks like it's not a delegate method.
Google is a special partner and this is why you get Google Maps integration, Google as the default search, etc... It only makes sense that they get preferred treatment in other situations as well.