There is absence of evidence of how whales behave around the bodies of dead whales over the long run. Absence of evidence is not evidence of absence (of intelligence), but it isn't evidence of presence either.
Which is why you use different lines of evidence. Is grieving behavior ambiguous? Sure. It is indicative, but also very alien. Put it in the plus column. Complex communication? Put it in the plus column. Evidence of advance planning? Put it in the plus column. Eventually you build a very strong case for erring on the side of sapience. It is done for dolphins orca etc. The case is very strong. For baleen whales, less so. On the other hand, their brains are the size of a couch.
Brain-Size relative to body mass is often used as a proxy measure for relative intelligence. However, that only works within certain ranges because the power of a brain is not a thing that scales like that. Being HUGE does not mean you need more neurons to control that mass. It is a function of structure--which we should not expect to follow the particular path ours did--and the number of neuron connections. This is why bird and octopus intelligence is freakish despite relatively small brains and less neocortex. Higher neuron density and a completely divergent evolution of brain structure.
But with a brain the size of a sofa? That goes in the plus column. We dont have the capacity to bring a bowhead whale into an aquarium though, so the case is just not as strong.
If the baleen whales communicate with language, and so do the orcas, I would think... put this way, if our tribal ancestors encountered orcs or dwarves or some other fantastic near-humanoid species, I would think that we would be able to deduce their intelligence from knowing that they can talk to each other, if nothing else.
When they are that close? Maybe. But not all languages are translatable. Orca and baleen whale communicate in vastly different fashions even though they both use sound. Different frequency ranges etc. Between humans and orcs, say, there is mutual tool use, maybe written language etc. You can watch them and they are similar enough to us that we can draw the analogy.
A human and an orc can look at eachother and think "Ok, they look like me, they write, they forge iron tools. I dont know what they are saying, but they are saying something."
Now, think about this as if you were an Orca. It is somewhat different. There is this HUGE thing there. It makes sounds, but you have no way of knowing if it is language. You have no way of knowing what it means. You cannot get these things in a controlled environment etc.
We can put a dolphin in a tank, teach them to associate pictographic symbols with actions, and then string together imperative sentences to one individual and watch it somehow communicate that command to another dolphin that could not see the initial command, and watch them do the thing requested. We can tell them make something up, they will hang out under the surface and we can listen in on the sounds they make, and then they will perform some unique acrobatics routine that is synchronized.
It took centuries of intellectual development before it ever occurred to us to do this. We took it for granted that we were the only sapient beings on this planet for thousands of years after we had writing.
As far as an Orca is concerned, that whale is just a thing that makes noise--just like everything else in the ocean. Just like, for us, chickens that we are pretty sure are not sapient at this point. How on earth is an Orca to know? If the language is sufficiently different, there is no way at all to distinguish it from noise. If chickens were sapient and used all their calls and clucks as language, we would not be able to tell they were even sapient until we brought them into a lab and ran some tests. Those tests have mercifully all come back negative.
I would normally default to stopping a cannibal to save their victim, even if the alternative is starvation for the cannibal.
Then you are trading one death for another. Net 0 in the utility column. Congratulations! You have accomplished nothing.
The difference is that you can offer the human cannibal a god damn sandwich and make it clear to the cannibal that it is edible and teach him how to make more if necessary. You might even be able to provide the needed materials *gasp and shock*. Unless there is something fucked in their head, they will typically be happy you did so.
The solution is figuring out a way to offer the Orca an alternative so they get to live too. Maximizing utility is often a long term thing. It is not always possible in the short term.