No sound expert here but these guys used very thin sheets of Styrofoam. People use foam for sound absorption, not reflection. It may be that both the camera and the sonar signature of an object are used to decide on applying the emergency breaking.
If styrofoam is essentially invisible to radar, then I see how the test really didn't prove anything. I've witnessed the TACC in action, but was just hoping to find some benchmark videos showing the effectiveness of emergency braking, then came across this video via google.