I never even mentioned your point about cars swerving into lanes, I was responding to your two absolutely incorrect assertions which were that it only works if ACC is active, and that braking must be initiated by the driver.My manual specifically states the system is limited when a car swerves into your lane, which is what the OP stated happened to him. I don't see any disconnect between my observations and yours.
Now if the car was ahead of him in his lane and braked suddenly, I agree the system should have braked independently. But that is not how he described the incident.
We can debate whether the system should be more sensitive to cars swerving into your lane. But my point is to say the manual clearly lists the limitations, something the OP could have known if he had obtained and read the manual. I'm just being factual, not trying to offend his sensitivities.
I don't for one minute disagree that no system will catch all possibilities, but for the avoidance of doubt, the BMW system is permanently active and does intervene actively if triggered.