Scientists say dolphins should be treated as 'non-human persons'

It is morally repugnant to mistreat animals regardless of the intelligence of the animal and being a scientist has nothing to do with making such a judgment, since it is not a scientific judgment but an ethical one. Thus, the new-found evidence discussed in the article serves only to dramatize something these scientists want because of their philosophical predispositions, not because of their special knowledge as scientists. If the logic suggested in the article were true, the converse might be up for consideration as well (e.g., that we can be cruel to deer because they are so dumb).

Science and advocacy are distinct and should be kept that way. There is nothing wrong with scientists acting as advocates but, when they do so, their views as advocates should normally carry no more weight than do those of others.

I could do without the political crusade though.

I don't think this is entirely true here. You've loaded the question by making an assumption, "It is morally repugnant to mistreat animals regardless of the intelligence of the animal" which not everyone will agree with.

In fact, I suspect the vast majority of people do not: every day millions of people knowingly eat meat produced from animals that were horrifically mistreated, despite the fact that they could have eaten something else, often for a lower price. Why? Because it tastes good, and a lot of people don't care about the suffering of cows.

Suppose the following moral axiom that some hypothetical person might hold:

"Mistreating any sapient being is equivalently morally wrong as mistreating a human."

Therefore, if scientists can prove that an animal is sapient, those who follow such a belief will agree that such an animal should not be mistreated. I don't see the problem with scientists arguing that an animal is sapient.

Anyone can advocate for dolphin rights, but, as scientists who have done research on dolphins, they are in a position to make ethical arguments against keeping dolphins in a zoo for public entertainment.

The rationale behind these arguments are grounded firmly in science. You have to be able to show the research that proves that one animal has no sense of self, while another (a dolphin) does, etc.

Everyone's opinion does not have equal weight.

I agree though that no one group has a monopoly on ethics and everyone can have an opinion of some worth.