Openpilot uses an embedded controller that is connected to the phone via usb to control communication on the CAN busses. It is what enforces the disengagement,

Openpilot uses an embedded controller that is connected to the phone via usb to control communication on the CAN busses. It is what enforces the disengagement, and the code on it follows the MISRA standard. The phone can send random commands and you should still be able to maintain control of the vehicle.