My best guess is that it is using the Siri keyboard speed-to-text feature and the app just does its own interpretation on the text?

I'd venture to say they got access to the API for this, similar to how some software companies were developing apps before the sdk was released to the world. Just a guess, based on the fact the article says they're the first car maker to integrate with the new apple technology.

I highly doubt that there will be an API for Siri. Nuance licenses their voice recognition sdk and they want to keep that revenue stream.