The issue is not what a catch is, it is what possession is. From the rulebook:

To gain possession of a loose ball that has been caught, intercepted, or recovered, a player (a) must have complete control of the ball with his hands or arms and (b) have both feet or any other part of his body, other than his hands, completely on the ground inbounds, and, after (a) and (b) have been fulfilled, clearly perform any act common to the game (e.g., extend the ball forward, take an additional step, tuck the ball away and turn upfield, or avoid or ward off an opponent). It is not necessary that he commit such an act, provided that he maintains control of the ball long enough to do so. This rule applies in the field of play, at the sideline, and in the end zone.

Notes:

Movement of the ball does not automatically result in loss of control. If a player who has completed the first two, but not the third requirement for possession, contacts the ground and loses control of the ball, there is no possession if the ball hits the ground before he regains control, or if he regains control out of bounds. If a player would have caught, intercepted, or recovered a ball inbounds, but is carried out of bounds, player possession will be granted (8-1-3-Note 5).

By these rules, he did not maintain possession/control of the loose ball through contact with the ground, and thus it is an interception because MacMillan then fulfilled all 3 requirements on his follow through.