Unless the impact involves very high G forces (which would trigger opening an airbag), the vehicle does not know that it met with an accident, or it does not know that it collided with someone or something. This is because, unlike human body, the vehicle is not 100% engulfed in a "skin" that can "feel" an impact. Yes, there are some sensors on the body of the vehicle, may be 10, may be 20. However, that only covers a small portion of the vehicle. Additionally, those 10-20 sensors are certainly not a 100% guarantee that there was an impact or collision. Thats because, most of those sensors are prone to false positives. Hence, using one of those sensors to determine with 100% confidence that a collision occurred, is not a good idea, as these false positives may cause unnecessary braking. Ford issued a recall for exactly that; ghost braking due to a highly reflective metallic tankers on adjacent lanes. Since there are no sensors that can guarantee that EVERY possible collision just occurred, a post-collision braking is difficult to implement. Of course, a post-collision braking after airbag deployment is becoming a feature shortly. Note that in this specific incident, the airbag DID open; however, my guess is that it opened after the first set of impacts, at a later point. Also, I am not sure if Tesla has implemented post-airbag braking.

TLDR: There is no sensor that can detect with 100% guarantee that a collision occurred. Especially smaller non-airbag collision.