Re: They kinda do and kinda don't
"It is possible very, very old versions might have done that"
The 16 year old ACC design in my 13 year old car will brake - gently - to open up a gap, if someone cuts it up and is moving slower than I am. For the same speed it will simply ease off the throttle slightly to achieve the gap and if they're faster it will maintain speed and let them open the gap up.
It it thinks it may need to brake hard it beeps furiously to warn the driver and everyone else in the car just before it applies the brake. If you're driving beside hedgerows it sometimes triggers a warning but won't brake. Ditto on large signs close to the edge of the road. (it's effectively a potential lane intrusion warning)
Newer versions are even smoother than mine, which is based entirely on lidar-in-the-bumper.