I knew they were smart. I didn't know they were sarcastic.
The dolphins are sarcastically reveling in the stranded doom of the two people on the island. Instead of trying to help the pair of people, they are making fun of them by replicating the island, tree, and people with an elaborate pose. This pose requires a lot of smarts to do, but, the people didn't realize that dolphins were smart enough to be sarcastic.