It is implicitly relying on the car in front to be driving at safe speed so it is maintaining breaking distance to it as if it will start stopping then. That is quite normal - most human drivers drive this way. You need 75m braking distance for 70mph. Nobody does that. Everyone drives closer and pays attention to what the other vehicle does as well as looking over it to what is AHEAD of it on the road.
1. The LIDAR and other hard instrumentation of the autopilot suite do not see past the car in front.
2. The software on the vis feed is nowhere near to do that level of analysis at present. You literally have to judge what the other driver has piled up in their rear window and make judgements based on partially obscured views and indicators/stop lights of cars way in front of you. We all do it instinctively after driving for a few months. Automated cars - not even there as this test clearly demonstrates.
3. We make additional decisions based on the car in front. I am actually expecting an Audi driver to switch lanes after the first blink of an indicator (still better than a teenager in a pimped up Saxo which will not even bother). I give it extra 10-15m just because the person in front is pretending to be in the cockpit of a Me109. The autopilot in the BBC video did not do that.
By the way, the BBC video is nearly identical to the reconstruction of the recent incident with the crash barrier in California. The sole difference is that it was a cardboard car, not a concrete barrier. The other variables, including an audi switching lanes at the last minute (coming out of a "non-lane") are identical.