Clearly, "not as though they were introducing anything that had been lacking in their predecessors" doesn't mean there are no statements in the Creed of 381 that weren't in the Creed of 325.
The Creed of 381 contained a statement that wasn't in the Creed of 325, but it didn't introduce "anything that had been lacking in their predecessors" because the statement in question was already part of the deposit of faith.
Once again: the doctrine concerning the substance of the Spirit was absent at all in the Nicene Creed; for this reason the Definition of the Chalcedone Council names the Creed of 381 as the doctrine concerning the substance of the Spirit:
"...it confirms the doctrine afterwards delivered concerning the substance of the Spirit by the One Hundred and Fifty holy Fathers who assembled in the imperial City"
...whereas the doctrine of procession of the Spirit
is already explicitly present in the Creed of 381.
Don't you see the difference?
Since that the Creed of 381 is not "modification" of the previous Creed, it's just another Creed concerning different matter (the Spirit).
As we have four Gospels, that don't "modify" each other but supplement each other; and sure we cannot modify neither of them as if they were "lacking" something.