But he hit the corner post.

If anybody says that to you, you know he has some catching up to do.

The All Blacks attack and Mils Muliaina throws a brilliant long pass to Rene Ranger who has an overlap as Gio Aplon of South Africa speeds across to tackle him

Aplon does tackle but Ranger gets the ball down in the field of play.

Before he gets it down, a substantial part of Ranger's body is beyond the touch-line and part of it hits the corner post.

Law 19 DEFINITIONS
The ball is in touch if a player catches the ball and that player has a foot on the touchline or the ground beyond the touchline. If a player has one foot in the field of play and one foot in touch and holds the ball, the ball is in touch.

No part of Ranger's body touches the touch-line or the ground beyond it before he touches the ground. He is airborne.

The cornerpost remains merely as a help to the assistant referees/touch judges as to whether the ball crossed the touch-line or the touch-in-goal line. If a player or the ball hits the cornerpost it is what happens afterwards that counts - as is the case with the goal posts.