AEB is not necessarily designed to prevent a crash. In some specific circumstances and speeds it may, but that's not the overriding design philosophy.The automatic braking happens at almost point blank of impact, and it will slam on the brakes violently.
I do know there was a very high probably of impact if the FCW didn't see the person pulling out of the left turn bay into the path
Think of AEB as an virtual crumple zone. It's designed to reduce the speed of the car at impact. If it can drop your speed from 40 to 20 prior to impact, that's a 75% reduction in impact forces and kinetic energy. Dropping from 60 to 40 is a 50% reduction in kinetic energy and impact forces.
It only kicks in when a crash is imminent (from a computer's point of view and reaction time, not a human's). And it's very, very conservative because it can be (no reaction lag for the computer), and so you don't get false alarms and shut it off.
Automatic emergency braking, also referred to as forward automatic braking, automatically applies brakes when the system detects that a front-end collision is imminent, to help avoid the collision or reduce the collision’s severity. - GM Website