I mean, sure we can. Language is a tool, we use it to perform a function (communicate).
"Cannibalism," as a term with a history, came into being in a world where there was only one sapient species: us. Historically, what the word actually meant was "Carib people." It comes from caníbal, a variant of caribal) because of the mistaken (and, generally, pretty racist) idea that the indigenous people of the Lesser Antilles consumed human flesh. But in any world where you only have one sapient species, the distinction between "person who eats members of their own kind" and "person who eats other sapient creatures" vanishes: all people who eat sapient creatures necessarily eat members of their own kind and vice-versa. It's a degenerate case.
We have no word in English, or indeed in any language to the best of my knowledge, for "eats other sapient creatures," because there's never been a need for one. We generally understand that that part--eating other sapient beings--is the morally relevant problem of cannibalism. That is, while there are other non-moral reasons not to engage in cannibalism (e.g. Creutzfeldt–Jakob disease), the primary problem of cannibalism is that it means eating a person, and (usually) the implication is that the cannibal killed that person specifically so they could eat them. It's sort of like how if you want to refer to a person as failing to show even a shred of compassion for other creatures, the word is "inhuman," which in a strict definition would not apply to elves or dragonborn (because they are, by definition, not human), yet words like this (calling an elf "humane," speaking of a robot's "humanity") are used quite frequently because, again, we've never needed to make a distinction between "any sapient creature showing due compassion for the suffering of other creatures" and "specifically human creatures showing due compassion for the suffering of other creatures."
Under those lights, while it may be a colloquial usage, the very word itself comes from a colloquial usage, so making some kind of hard absolute stance of "this word means THIS thing and absolutely nothing else" is a bit specious. "Cannibal" is the closest term, both in terms of the physical act, and in terms of the moral ramifications of the act. Yes, a new word could be invented, and if it became important enough, I'm pretty sure people would try. Consider, for example, that there was no word at all in Latin for what we now call "cannibal," yet we know that they had a very strong taboo about not eating people! (To the best of my knowledge, the best you could do is a circumlocutive phrase, or simply borrow the Greek word anthropophagos, very literally "man-eater.")
Ironically, Old English did have a native word for "cannibal," but if we were going to follow your linguistic prescriptivism, that word would be wrongly used. Because that word was surprisingly readable to us today: selfæta, which very literally meant "self-eater." But a cannibal doesn't eat himself, he eats creatures like him. It is that sense of "likeness" that this looser meaning of "cannibalism" refers to. Not literally eating creatures of one's same species, but eating creatures of one's same moral kind, that is, sapient ones.
