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.
It seems to me they are using it, but as you mentioned navigation may not be the purpose. There may be none. It may just be that insects that do this congregate and reproduce thus it’s just a mate finding thing that’s evolved. Since insects often have short lives, I’d guess it also explains why they’re always doing it (they need to mate asap).
Might also explain why they don’t go directly to the light but eventually end up there circling it erratically, increased odds of bumping into a mate.
The paper mentions that the insects enter stall conditions, maybe due to the banked orientations, sometimes and then recover. This would explain erratic behaviour without any mate present.
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.