"We have no way of knowing the long-term effects of this toxicity"

Is someone arguing against that? I did not see it anywhere here?

Considering the volumes involved, I suspect the impact of boh the oil and dispersants will be seen in organisms for some time.

The point is that the dispersants used at source in this case worked to do what was intended (outside of long term biological uncertainities). On the sea surface they did not, and it was mostly discontinured.