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.
That doesn't look anything like a car, person, or bicycle. The Mobileye camera system is smart about what it sees, and this looks like none of those.
Styrofoam is not reflective of radar waves. What they built themselves is a wall of stealth material. They needed to coat it with something radar reflective.
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.