Well, prior to this if a penalty kick to touch was kicked before full time, and time expired as the ball was in flight, before it went into touch, technically that would be full time, right? Now the time left when the ball leaves play again is irrelevant - if the kick was taken before time expired, the game isn't over.
I wonder if there'll be a similar level of attention to such decisions at the end of the first half. It seems that quite often half-time is called when there was a scrum awarded before 40 minutes, etc.