An underhanded grip involves supination of the wrists, which is affected by ‘uncrossing’ the radius an ulna such that the entire length of the radius lies on the lateral side of the ulna. This is performed by the biceps brachii, brachioradialis, and supinator (longus)—only the first of which can act as an agonist in the pressing movement, being involved in flexion of the humerus. However, (because it is bi-articular), its change of length during the press is minimal. Supination may cause a very slight change in the angle of the ulna relative to the humerus, but since the elbow is a hinge joint, this is insignificant. The humerus is otherwise unchanged in direction and orientation, and as such, the supinated grip does not alter the bio-mechanics of the press other than altering the involvement of the biceps brachii.
Due to a physical limitation to the amount to which the wrist can be supinated relative to the humerus—according to Michael J. Alter in Science of Flexibility, around 0° (that is, normal to the frontal plane), but in practice, perhaps ±10° relative to that position—a supinated grip does limit the width by which we can grip the bar. Therefore, the underhanded grip does essentially enforce a narrow-grip press, which thereby activates the clavicular (‘upper’) portion of the pectoralis major significantly.
We can reasonably conclude that it is the width on the bar that the underhand grip enforces, and not the orientation of the wrists that has brought about the belief that the it activates the clavicular portion of the pectoralis major to a greater degree.
I hope that makes sense.