I argue that H 6 should be taken as Aristotle's clarification on the causelessness in the unity of the parts of definition. In H 6 Aristotle is concerned with a general metaphysical problem affecting - threatening - his theory of substance at two major points. The unity of genus and differentia in the definition of form has to be accounted for without appealing to a unifying cause. If it were not accounted for, form would not be the primary cause of being and thus not primary substance. The unity of the parts of the definition of composite substance also has to be accounted for without appealing to a unifying cause. If it were not accounted for, the definition of composite substance would be merely a formula referring to the material parts, and wouldn't be the formula of an essence. And the composite substance would not be a substance.
Kim, Hye-Kyung, "Metaphysics H 6 and the Problem of Unity" (2003). The Society for Ancient Greek Philosophy Newsletter. 400.