I think legally they would need to require using an app for their back view camera. All new cars in the United States after 2018 need one and I don't see how it would work without using the phone/tablet as a display.
The article says the rear camera feed will be displayed on a screen behind the steering wheel which doubles as the speedometer and charge display.
That seems like a clever and reasonable compromise to comply with a particularly bad regulation.
I don’t have a problem with the regulation. Slate is trying to build a safe, affordable, electric vehicle. Having one display for all information seems like a good way to save cost.