Actually the authors point would stand even for a perfectly smooth window pane. The point is that as the thickness increases the length scale over which light g

Actually the authors point would stand even for a perfectly smooth window pane. The point is that as the thickness increases the length scale over which light goes from constructive to destructive interference reduces, and as this happens you end up sampling more and more uniformly colours from a uniform spectrum. At some point our eye can no longer tell that we are only seeing n monochromatic wavelengths of light. Okay not monochromatic, but very narrow bandwidth peaks