The point of the story of Limhi and Alma was their miraculous salvation after all of the tribulations. The wickedness of Noah was so great that they should all have been destroyed or at least in bondage “were it not for the interposition of their all-wise Creator.” Even without a king, God is still in charge of the universe. In spite of a king, God can still save his people.
Mosiah’s argument here is subtle, but important. He has proclaimed that the ancient connection between God and king is good. What happens when half of that conduit is severed? Mosiah describes that very event in the story of Noah. Wicked King Noah was clearly not proclaiming God’s will, but God was able to take care of his people even in the absence of a righteous king. By proclaiming God’s victory here, Mosiah establishes the continuing importance of God. Removing the king will not remove God.
When Mosiah proclaims that God could intervene only because of the humility of his children (verse 20), the point is made that the re-establishment of a king (Limhi) did not automatically recreate that conduit between man and God. The person and position of the king did not cause their deliverance, but God himself. Thus God may continue to guide (and save) his people even in the absence of a king.