Nothing prevents them from having access - it’s just a matter of reverse-engineering the messages transiting on the various CAN buses the radio has access to.

Zenec for example makes units that are able to display & control HVAC and parking radar on VAG Group (Volkswagen/Audi/Seat/Skoda) vehicles, pulling the data from the CAN bus.

In the authors case he’s reusing existing manufacturer-provided libraries that handle CAN message parsing but there’s no reason you couldn’t do it from scratch with enough effort.