That's not really how it works even when a new car goes into limp mode. Usually what happens isn't that they limit the spark. It's that the PCM (Power train Control Module) provides a ground to various systems (which are always powered via the battery/alternator charging system). When the PCM or ECM (Electronics Control Module that monitors network traffic in the vehicle) detects missing voltage from a monitored network sensor, or too much voltage from a monitored network sensor) it will put your vehicle into limp mode to prevent more damage that would occur if you keep driving. For instance if your camshaft sensor is providing a reading that would suggest it's not spinning or is "stuck". That could do internal damage to the engine if the vehicle continues to be driven.

But even so what they meant was that disabling a vehicle in motion is actually dangerous to the driver, any passengers and any other people driving or riding on the road.

Additionally, if the government can do it, that tech could be used by a bad actor for the same purpose and that's just not going to fly.