I'm really not sure but I think there is an error in the article here. I don't think the image antenna has to do with reflections. For instance think of a ground plane that is angled downwards at 45 degrees. If it was about reflections this would really change things. Instead I think it's about how the electric field can form around the antenna. The ground plane gives the field something to form to. The shape of the field is identical to if a dipole was used, it is identical to if the image antenna was real. Without the ground plane the field would have to terminate maybe at the antenna connector and it wouldn't have a good shape anymore.