The person you were replying to was talking about adaptive cruise control and automatic breaking. The majority of cars on the road do not have those features. Very different than plain acceleration and braking and much more likely to have glitches.

Braking systems are controlled by a dedicated embedded software system that's extremely well-understood and well-tested (probably even formally verified). All it does is check the ABS (and anti-skid) sensors and moderate the braking pressure to avoid locking the wheels. It also has a fail-safe system that gets rigorously tested, e.g. if a sensor fails. All possible values of all inputs are known, and the configurational space of those inputs is small enough that you can exhaustively test correctness.

It's plainly obvious that a braking system controller software is a very different beast from an autonomous car software, and I have no problem understanding why someone would trust th former and not the latter.