In 1979, Ixtoc I released 3.3M barrels into the Bay of Campeche (DwH released 4.9M).
By the time DwH happened we had already learned that
exposed, high-energy habitats bounce back;
sheltered and deep-sea habitats do not.
As someone who studied marine science this is right. Anything deep sea grows slowly, needs to conserves energy and is likely very old.
Also there's comparatively little oxygen available and almost no currents to mechanically break up large masses of tar.
Out of all fossil fuel extraction methods, deep-sea drilling is the worst.