That explanation is just lazy journalism. Sure, some of Siri's OS integration might rely on private APIs, but a lot of this stuff is exposed publicly .
It also obscures the real reason that Google Voice doesn't do this stuff, which is that it's trying to solve a different problem to Siri. Google are (unsurprisingly, when you think about it) making a voice interface to a search engine. Apple are making a voice interface to a phone. The difference in capabilities reflects their different purposes.
The problem with Siri is that it isn't clear enough about what it's there for. The skeuomorphic 'personal assistant' thing really doesn't help either. Maybe Siri would have been better received if it had just presented itself as a new way of setting appointments, etc.
1. For example: http://developer.apple.com/library/ios/#documentation/EventK...
2. If it did this stuff, I bet it'd take you to your google calendar whether or not the iOS APIs are public.