Nope, CANBUS is an open protocol, if you have a real CANBUS reader (not one of those $13 ELM junk) you can read every piece of data coming across the bus. A honda civic is the cheapest car you can buy and when I say cheap this also means they went cheap on security too. Newer e.g. Audi's and Mercedes' have since separated critical components to a secondary bus (which you can access via the ECM under the hood but its no longer as simple as plugging into the ODB2 port).
My question is how is steering done? If I had to take a guess, they are manipulating individual brake controllers via the ABS system but that won't allow the car to make a 90 degree left or right turn (efficiently). And a Honda Civic is DEFINITELY not steering by wire yet (or maybe it is?). Regardless of the method used, it's still fucking scary to think someone on the road next to you could be using one of these instant-death devices. Why do I call it that?
A year ago I got the bright idea to start hacking lane assist into my mid-2000s vehicle. My wife's car has it and I was driving 45 minutes one way to work. I wanted an excuse to skype while driving, to be honest. Found the CANBUS is wide open and for $600 I could get a device that would dump out and write to the CANBUS. Started dumping data as I drove and eventually isolated the individual brake commands. Now, I never got around to sending brake commands because of a very scary article published right here on Slashdot. It was about liability of self-driving cars. Immediately I had a thought of my system spazzing out and locking up my rear brakes while doing highway speeds. Not wanting to risk my cushy lifestyle I put the raspberry pi and CANBUS writer on a shelf and started looking into the Infiniti Q50 (don't buy a pre-2015 model folks, the steering wheel is dead to the world and the car wanders on its own).
Shame on comma.ai for releasing that kit, I'm actually even more scared to drive than I used to be and it's only going to get worse as more entrepreneurial car-hackers hop onto the road.