actually he's not. Joint motion is not dictated by the positioning of body, rather the motion of the joint, hence JOINT MOTION. For example, flexion at the elbow is still flexion at the elbow no matter where your body is-IE: a bicep curl is still a curl even if you are upside down with your legs over your head as long as flexion at the elbow is still the primary joint motion. Horizontal flexion, in the case of the press is horizontal flexion no matter where the rest of the body is as long as the arm moves horizontally across the body.
And if there is no clavicular head of the pec, or clavicular pec attachment with muscle fibers functioning for shoulder flexion, then millions of people studying anatomy are being terribly misled. But, I'll make sure to tell them the truth, now that I know.