Yeah, I would have doubts that they come into contact with humans (or our fishing boats/nets) often enough to have words for us. But swear words are a pretty attainable language feature...
I mean, there are real projects. I know that the orca population near Vancouver is a focus of decoding mammal language.
They have recordings going back like 30 years and log books to connect the calls to orca behavior. Last I checked (a few years ago) they were pretty far with unsupervised learning on the audio data and we're going to tackle the (barely readable) logbooks next.
I doubt we are far enough to train ai for animal-human translation on more than a conceptual level, and I doubt that I would hear about it from dolphins for the first time.
I expect a widely covered story of translating dogs' barks (or cats) first, and not in a "Hello Human, Welcome back home, I missed you. Please give me food" way (which would be probably fake) but just "Friend! Joy. Hungry"
And I don't know how we could scientifcally differentiate a slur from a descriptive name on that conceptual level.
What I don't doubt is that dolphins have slurs for humans.
I kinda think you might be right. If you have an animal that has nouns wouldn't one of those nouns be for the biologists that keep hanging around it and interacting?
Can already kind of do that with dogs. They're like a Twister board with big buttons the dogs can push. As you say, not whole sentences but "human, leave, dog, sad" is essentially them learning our language.
It's debatable if they really understand it, or if they just press the buttons that make their humans happy. And dogs are really good in spotting even the tiniest clues in our body language
Carl Sagan didn't intend for the researchers to give the LSD to the dolphins. They were supposed to take it themselves to see if it helped them understand the dolphins. May have still ended up in a dolphin wank though.