The particular warning in question states that ACC is not designed for use in stop and go traffic. That's not a legal CYA (though I'm sure it could be used in that way). They are explicitly stating it was not designed for that purpose so don't use it for that purpose. ACC was not intended to be an aid to avoid collisions (that would be "Forward Automatic Braking", which does have a legal CYA).This is why GM, along with every other auto maker, explicitly states in their owners manual that these aids are not to be depended upon for safe driving. It's to prevent the injured party from going after the deep pockets of the auto maker vs. the shallow pockets of the driver of the other vehicle.