Apply Occam's Razor. User or mechanical failure are much more likely than his car being hacked.
The story talks about a *wired* port by the parking brake. That would mean the attacker was in the car, or a remote device was attached, which investigators would (or at least could) find. It also only addresses a specific Ford vehicle, which has no relationship to a Mercedes.
Significant user failure would seem to be present. Options are available when the brakes don't work. Downshift. Turn off the key, let the engine stop, turn the key on to unlock the steering wheel. Spin the car. Even hard maneuvering will bleed speed off. Ask any racer. Turn the key off, let the steering wheel lock, and have a slower speed impact into a fixed object.
The option of driving as fast as possible, and dying in a fireball is the poorest choice. A conspiracy is one the must unlikely scenarios, only slightly better than alien abduction/intervention, and poltergeists taking over the car.
I'm kind of fond of the alien theories.
If it were the feds, wouldn't it be easier to pay a thug to do a random carjacking? A home invasion gone wrong? Shot by SWAT in a drug raid at the wrong address? There are a million other ways to remove someone without needing a high tech solution that doesn't exist yet.