The very clever Beluga may be a special operations worker from Atlantis. It is possible it was a domesticated animal 50,000 years ago bred by humans to help with tasks underwater.
Neither one of the reporters realises this is freeform mimicry, very different from a lyrebird or a parrot. The dolphin is not imitating speech it has heard as a kind of echo, it is imitating human speech itself.
A diver in a suit like that worn by the fish-god Oannes could have an annunciator that would send his own speech through the water to the Beluga and the Beluga could speak back. They might not discuss philosophy but they might be able to communicate specific actions to each other. "Go and fetch me that tool," or "Hold this while I screw this down," or "Haul that net of fish back to the airlock."
These animals love humans, seem to want to interact with humans and appear to be equipped with all kinds of special qualities that enable them to work together alongside humans. This is true of so many marine mammals, like the dolphin and the seal.
Crazy, you're thinking. I'm thinking, why would a whale ever need the capacity to swim backwards? Moving around complex human architecture underwater as construction duties, the beluga would need the ability to swim forward into tight places and then back out.