That isn't the issue. Everyone agrees that the platoon advantage exists. The issue is whether it is a product of the skills of the players, or (from the standpoint of the hitters) simply an inherent advantage in the game, applying more or less the same to everyone.
Every baseball broadcast will include some statement such as "Harry Armpits is hitting .380 this year against left-handing pitching!" or "Johnny Fattbutt hasn't had a hit yet this year against a southpaw." Probably the broadcast will have MANY statements of this nature.
But I would argue that all such statements are a waste of time, because there is no reason to believe that hitting well against lefthanders or righthanders is an individual skill. It is simply part of the game. If a player hits .380 against lefthanders, or if he doesn't have a hit against lefthanders, that's just something that has happened. It doesn't mean anything.
Of course there are limitations to this generalization, among them (1) Adam Jones, (2) switch hitters, (3) the fact that a player who has a high percentage of three true outcomes will have a larger platoon differential than a guy who just puts the ball in play, since there is little or no platoon difference if the ball is put in play, and (4) certain exceptional players who, after years of being platooned, lack confidence in their ability to hit against the wrong kind of pitcher.
But the damage done from the false belief in these platoon splits for individual hitters is vastly greater than the damage that could be done by a false belief that there is no individual effect at all. Real mistakes are made every day in lineup construction and in the handling of players, the shaping of careers, based on a baseless and arbitrary belief that this particular player has no platoon split, or that he has an exceptionally large platoon split.