Actually this "no spill over"-design was innovated by Rory Byrne with the championship winning Benetton B194 :
Rory Byrne later improved that design on the WCC-winning Ferrari F399:
It's probably no coincidence that Ferrari's design team listened closely to the advice of someone named Rory Byrne on the car that produced the last evolution of such sidepods.
David Sanchez, the creator of these sidepods on the McLaren MP4-26, like many F1 designers and engineers, copied one of Rory Byrne's numerous ideas. Since David Sanchez is now an aerodynamicist and chief of vehicle concepts for the Ferrari team, advised by Rory Byrne, one could say that this evolution is almost a logical consequence of this constellation.