Is there some way that this or similar can be copied by all automakers, or is this all patented by Kia/Hyundai in such a way that most improvements are completely locked in?
Assuming the technology has actual value, Kia/Hyundai would likely be more incentivised to license it to other vehicle-makers, than to keep an exclusive license for themselves.
Especially considering how their name and brand are in the dumpster, and likely won't recover. Their cars will be targeted for break ins for a long time to come due to how viral the vulnerability is, regardless of whether or not the model is actually susceptible to it.
Manufacturers often buy gearboxes from suppliers instead of producing them. Hyundai does have Hyundai Transys as separate company for transmissions, and they would likely be happy to sell complete devices, not just licenses.