By saying a thief would have to understand how a computer exploit works, it's saying a thief needed the equivalence of an engineering degree to drive away with a car before computers entered the equation. Exploit discovery, maybe, but it doesn't take much to execute packaged tools you bought on the blackmarket.
Whatever the reason, stealing newer cars is harder. That increased difficulty translates into decreased theft rates.
For now. Once they get out of circulation only newer cars will be left and at that time it might be that thieves could buy devices to hack into these cars just as you can rent botnets today.
Not for long.
But I really doubt that all the thieves out there will learn fancy technology so they can steal newer cars. A few will, but most will find other things to steal.
Right now, popular new cars are stolen in amounts of hundreds per year in the US. Older cars (like the Honda Accord) are stolen by the tens of thousands. That's not because older cars are more valuable, it's just because it's a lot easier.
If you're worried about safety, buy a car with a good crash safety rating. You're far more likely to get into a normal crash due to bad human drivers or mechanical failures than you are to be hacked.
If you're worried about the financial loss from a crash or theft, your best protection is good insurance.