I think a lot of the comments are also missing the fact that the moon is an unreliable means of navigation due it not always being present the whole night (or at all), changes brightness, moves around the sky and can be covered by cloud. It is unlikely then that insects would specifically evolve to use it as a means of navigation.

The paper states that it is the general brightness of the sky, even at night, compared to the ground that is the point of reference. So insect point top side at diffuse bright area and bottom side will be parallel to ground.