Natural processes are at a much greater disadvantage when the oil remains at all depths - and in large concentrations. Which is the case here.
Sorry, one hundred and eighty degrees WRONG. The more spread out the oil is, the easier it is for natural processes to work. The only processes for which this is not true are sun-induced photochemical reactions that happen in the layers closest to the surface. But these comprise only a tiny fraction of things that reduce spills. Most reduction happens by bacterial action. The more spread out the oil is, and the smaller the particles it exists as, the easier the bacteria can work.
Dude, I'm a chemist. This is my SPECIALTY.