While dolphins no doubt are intelligent, the belief that it is second only to humans (including language, complex social structures, etc.) is simply not true.
Note that some media outlets reporting this are taking away the wrong conclusion. Gregg points out that some aspects of dolphin behavior (attributed to superior intelligence) are shared by many lesser life forms that nobody would consider intelligent (like goldfish and mealworms), leading some to conclude that he's placing dolphins among them on the intelligence ladder. I don't get that impression from the article, and Gregg himself has said that he does not hold this belief.