A smooth surface is good and for this example the reference. Some specific roughness might be better at a certain speed. But on another speed, this exact roughness will slow down more than a smooth surface so it will be difficult to gain more speed. You need the exact speed to obtain the optimal gliding for a certain roughness.
Polished might be better in that specific test. But the sailor will influence the endspeed much more than a polished or non polished something.