More than a year ago, Google announced that the Assistant Driving Mode was supposed to replace the Android Auto interface on phones. It looks like the company ran into significant problems with the software because it took it many months to make good on its promise. The new Driving Mode is only now starting to roll out to more people in the US as an early preview.
Google already has an in-car navigation UI in the form of Android Auto, as well as a future replacement in the form of the Assistant Driving Mode, but it seems to be implementing a very similar concept in Google Maps. We've seen music playback controls show up in Maps navigation before, but never the Android Auto-style buttons and home screen.