It is their ability to understand sentences of sign language that astound though, with a sentence like “touch the frisbee with your tail and then jump over it” returning just that from the dolphin. This proves more than just rigorous training is the answer, but an understanding of what we are asking of them.

Animal intelligence is one of the most interesting fields of research and exploration. Understanding what intelligence might mean independent of the human context will first and foremost teach us something about human intelligence

What makes us so damn sure we are the model for intelligence?

I have no idea, but it seems it has something to do with the belief that everything is related to us. Actually this is very basic to our perception, the very mechanism of observing implies an observer.

A quantum theory?

this behaviour has been shown in other cases with animals the reward punishment condition
first the subject does their thing unimpeded
then once rewards are introduced it does as little as possible to get a reward
also works in human society
pleasure and pain

In this case, the beast is strategizing and methodising to gain his treat when none are available. It also had the forethought to 'Hide' the litter. Now the beast can pursue its own reward schedule instead of being rewarded by happenstance.
An animal trained to retrieve will retrieve. I have Labs. They MUST go fetch.
This dolphin Is hoarding and rationing its retrieval to meet its own schedule. Nobody taught this behavior it was learned insipiently.

Dolphins do much more than that indeed. Read Gregory Baetson's work on dolphins. In his work, he figured that dolphines are capable to understand contexts and operate accordingly.

dolphins are said to be highly evolved
http://www.crystalinks.com/dolphin.html