Well, if Kellhus figured it was easier to do with the actual Diamos, then likely it really was, since no one seems to ever have had more Gnostic power than him.
Perhaps it's a case that the Gnosis' "formal" power is exactly that, too formal to do much in the abstract realm of the Outside. Perhaps even, Yatwer (and the rest of the 100) are not even "formal" entities, as we would imagine, for example, the Solitary God would be (was it to exist).