I realize you can just answer, “mysterious aliens beyond our understanding,” but why would a space probe have a mandate of either talk to a whale for 30 seconds or destroy Earth and disable everything in its path on the way to Earth?
Why? What does that achieve?
It probably should’ve been dolphins, just as a nod to H²G², but also because I’m not sure the scale of that Klingon BOP ever read as big enough for two whales to me.
But, some possible suspects: whales (maybe they are superintelligent, pan-dimensional beings and the business with the krill and the singing is just a front), whale keepers or collectors (maybe they’ve stocked Earth with an important livestock that they want to protect from the apes, or see TNG S3E22 “The Most Toys”), whale descendants (see VOY S3E23 “Distant Origins”), or a species that has determined that loss of the whales indicates humanity has become irredeemable (or a corrupted timeline).