Ed,
Maybe someone from Hanse Design will clarify that statement. Certainly, back east where the soil is heavier, gathering bunkers with the land contours feeding into them create drainage issues as they also gather all the rain water.
However, I think there is a difference between contours feeding into a bunker and having a bunker encased by taller grass which impedes the progress of the ball.
Plus at Rustic Canyon, the sandy soil makes drainage a non-issue.
Cheers,