Siphoning is the act of using a tube with different levels of pressure at each end to transfer a liquid from one end to the other. Using atmospheric pressure or different elevations is just one method of siphoning.
The greater level of water pressure at the bottom will push the oil (mixed with water) up to the lower level of pressure at the top of the box/tube contraption, where it will be whisked away in a controlled fashion (rather than dispersing into the Gulf).
It's much like sucking water out of a glass with a straw. The greater pressure from the water at the bottom of the glass pushes the water up the straw to the lower level of pressure inside your mouth. This is also siphoning.