It's not as if embedded systems work the same way as the jumbled mess of technology that web servers use. It's perfectly possible to firewall off core system functions into a separate network and separate individual processors for each function. Existing vehicles today have already a strict hardware firewall between the CAN bus[1] and the IVI[2] system to prevent bugs in the more vulnerable and less strictly designed IVI software from sending data that disrupts safety-critical systems.

[1] https://en.wikipedia.org/wiki/CAN_bus

[2] https://en.wikipedia.org/wiki/In-car_entertainment