On the tele, it looked like well polished wood. Like a gym floor or basketball court. I hope I'm wrong because it wouldn't be the grippiest of all surfaces for a bike. Looking at the photo though suggests that it's a different surface to the rest of the flooring so perhaps something a little more bike-friendly was put down.
Prudhomme was interviewed afterwards, expressing an interest. Perhaps we'll see it in the TdF in the future.