It’s very deep, so as it rises it separates into different components, some of which diffuse into the atmosphere, some of which float (but very widely dissipated due to mixing with all that water), and some of which sink (tar). At least I think that’s what they’re saying. So, a deep-water spill where it travels a mile to the surface is very different from a situation where it spills directly onto the surface.
This post is exactly correct folks.
Just to add, those components that are light enough to make it to the surface are typically light enough to evaporate before making it to shore.