I think when a Sahuagin eats an Aarakocra it isn't cannibalism, but it's very close to the line. I don't think a Lizardfolk eating a human isn't "cannabalism" but it is crossing a major line. A dwarf eating an elf is cannabalism, though, IMO. Sure, they're different races/species, but a fleshy creature eating another fleshy should count as cannabalism.
By the reasonable definition of cannibalism, that of a sentient eating another sentient, yes, sahuagin are a cannibalistic species. In fact, if I remember correctly, sahuagin have been depicted as cannibalistic by any definition, by being perfectly willing to eat their own (the strong eat the weak). Sahuagin are an embodiment of our fear of sharks, and while sharks aren't themselves cannibalistic (I'm fairly sure), our fear of them as voracious predators makes the mythic shark so. Sahuagin culture doesn't define this act as wrong or evil, but sahuagin are one of those D&D species that are considered either inherently evil worshipers of an evil god, or at least a very alien species that doesn't play by the rules of civilization.
Does our portrayal of sahuagin need to change? Do we need to reexamine them as some of us are pushing for orcs and drow to be reexamined? Are sahuagin people? I actually think that these are good questions to ask of ANY D&D fantasy race, but the answers don't always have to be the same. Sahuagin are a sentient species, they are people, but they are also quite classically alien and removed from the morals of humanity (demihumanity). Orcs and drow are a lot closer to humans than the scary sharky-fish-people.
Your larger point, that a dwarf eating an elf is way more squicky than a sahuagin eating an elf (or dwarf), holds true. The closer a fantasy race is to humanity, the, well, more human they are and the more problematic it becomes when we dehumanize them to make them almost-people that are okay to hate and kill without moral qualms. And the squickier it gets when a near-human race is described as cannibalistic.
Conversely, the more monstrous a race is, the farther away from humanity it is . . . . it becomes less problematic to see them as monsters. It becomes less squicky (although not less scary) when they break norms like cannibalism. Of course, this is fertile grounds for some good sci-fi style storytelling. If sahuagin are unabashedly cannibals and have no issue eating humans, elves, dwarves and even other sahuagin . . . does that make them monstrous enough that our "heroes" can kill them on sight without much worry or moral quandary?
As is often the case, I love the way Eberron handles the sahuagin. Keith Baker, your gift to D&D never stops giving. Sahuagin aren't really portrayed as evil, but more as a terrifying force of nature. If you are going to cross the sea by ship, you have to be prepared to deal with the sahuagin. But those interactions aren't always violent, the sahuagin can be traded with, and hired as guides and guards. They are a people worthy of respect as any other, but a people you must be wary around as their cultural norms make it okay to eat you! Some sahuagin have no compunction about raiding sea vessels come dinner time, others realize that there can be more to be gained by not immediately eating the surface dwellers and by treating them as, well not equals, but beings worthy of peaceful interaction.