Another way of stating the same basic point: the Arians, in arguing that “unbegotten” was a predicate of God the Father according to substance, were confused about a basic point of grammar: to predicate “not-X” of Y is not in fact to predicate anything at all of Y; it is only to foreclose the possibility of a particular predication, namely, “Y is X.” Saying, “Y is not-X” doesn’t then grant us any positive knowledge of Y whatsoever.
Augustine offers a helpful analogy: if someone tells you that a certain animal is not a quadruped, could you on that basis form any positive statements about that animal’s feet? No! (V.7.8) All you know about the animal is what it doesn’t have. Calling the Father “unbegotten” is like that: it is meaningful only as apophatic theology, and so it quite precisely is not a predication according to substance.