
Theory and practice are different
From a quick read of the research paper, it seems that all the hacking was done by injecting extra CAN packets on the car network, no code was actually changed on any of the ECUs. So to actually "hack" the car you have to have another deviced attached to it.
I don't think that it would be worth the effort for anybody to hack a car, there are much easier way to do similar kind of damage.