I don't see the flaw in my logic, no:
-- If the cup had a creator, we could know this, or not. If we knew it, then we could know something about that creator, even if it's not a particularly profound knowledge: "God, or the creator, is skilled enough to make this cup." So to an extent God would be knowable, and therefore not unknowable.
-- If the cup had a creator, but we could not perceive this, that would mean that we had conceived of a plausible alternative to the idea that the cup was made (or alternatively, that we weren't paying attention -- but we are paying more attention to the Universe than ever before, so...). So there exists a possible history of the cup that does not need a creator, that it just was. In such a history there is no way to distinguish between the cup being made, and the cup just appearing, so that to all intents and purposes the creator is irrelevant -- because you don't need him.
I do not say which of these is true, but: If God is unknowable, he is irrelevant; and if God is knowable, he is testable -- and, indeed, can be shown not to exist.