So that's a human arm and Rumia isn't a human... Thus it's NOT cannibalism
While you are correct, for tagging purposes we'd want to still label this as cannibalism to keep these thematically similar images together under one tag.
The term cannibalism need not specifically refer to same species eating same species. Fireflies of one genus eating fireflies of another genus is also referred to as cannibalism, so cannibalism in a broader sense could mean similar eating similar. Any species of human eating another species of human would also likely be referred to as cannibalism.