You can't get a polygon with a hole in it in a Voronoi diagram. He's starting with a voronoi diagram and then combining adjacent polygons of the same "color". You may then get a polygon with a "hole" in it. Example: A nine by nine array of points, where the center one is red, the eight adjacent points are all blue, and the rest of the points are red. If you take one of the blue points and merge all adjacent blue polygons until you run out, you'll get a small red polygon in the center (originally in the voronoi diagram), then a polygon surrounding that one that's blue, etc. I believe that's what was meant.