I think when it says that missiles in the coronal plane are more lethal than in the sagittal plane, it means that the gun aimed at the side of the head going to the other side of the head (coronal plane) is more deadly than if the gun was pointed under the chin or in the mouth (sagittal plane). I don't think its referring to literally aiming for the coronal or sagittal suture.
Its important to note that the study lumped in the mouth and under the chin as "sagittal plane" and that there were many more survivors submentally (under the chin) than intraorally (in the mouth). However, this goes to show that no single location is 100% successful. From the exit wounds or bullet fragment locations, it seems that in these unsuccessful attempts, the gun was aimed too much towards the face/ front of head rather than closer to the back.