It's very easy to imagine what they'd do with the battery in an electric car. They would do the same thing they did to the pager: put a bomb in it. You cannot "detonate" a lithium ion battery with remote code execution unless you have tampered with the components surrounding it.

It's not so much about the security on vehicles and more about the practical limits. Thermal runaway is only going to happen if you manage to disable the battery's shutoff safeguard (which is usually replicated in hardware and firmware). Unlike phones, you can't really heat up the CPU and force the battery to take the heat. You would have to manipulate the drivetrain in a way that unnaturally damaged the hardware, something a driver would notice almost immediately.

Getting the conditions right for a software-based exploit is a circus act compared to installing a plastic explosive and remote detonator outside the battery stowage.