As is typical, Grok will “evolve” as more info becomes available. Grok now says maybe intentional dumping, maybe a large spill. If these spills repeat, the answer will become more clear. It’s also possible it was done as a message, as this slick will end up on the shores of the GCC countries (my guess, not GROKs).
Bottom line is, there is no evidence either way at this point.
Good place to apply the 48 hour rule.