That is the key, 'intention'. God has every intention of carrying out what He said.
He, however responds to how man handles that warning!
Thus, even God's warning to Moses is real, and Moses handling it is real, God knows the outcome, but since Moses is free, the outcome could have been different.
That Moses chose to act in a way that moved God to 'repent' does not negate what God intended to do when He warned Moses.
God intended to destroy Ninevah and would have unless they had not repented.
That God knew what they would do, does not lessen or negate God's intention when He warns them, because in real time He is giving them a choice and will Himself act on that choice.
Nothing God says in time is 'untrue', God knows the outcome of the choice that man makes and what He will do also, but the choice has to be made first in time, hence the dynamic nature of the system.
God is seeing how His interaction with mankind 'plays out' and that is what is 'foreknown', but not a foregone conclusion until God sees it happen in time as real.
But that's also part of the problem, ftD. If God intended to do as He said AT ANY POINT, then God intended to go against his already SPOKEN promise that the Messiah would come from the tribe of Judah, not Moses' tribe of Levi.