Interesting stuff, but just measuring frontal area is probably an oversimplification. For example the drag of a streamlined body and a flat disk are quite different. They probably make reasonable assumptions about the shape of the bike and rider, but probably not about the bike, rider, and trailer. They may not make good assumptions about the bike, rider, and pannier combination either.

My best guess is that a bike with a trailer has less drag than one with panniers, but it is just a guess.

Another issue is if you group ride, the trailer is hard to draft effectively. I suspect that the trailer drafts the bike it is attached to pretty effectively though.