Knowing the Lord refers to more than just knowing his name. It is also a recognition of duty toward him, to know that he is the true God, and to have a personal relationship with him. The Egyptians would bring offerings, as a sign of their true religion.
21 And the LORD will make himself known to the Egyptians, and the Egyptians will know the LORD in that day and worship with sacrifice and offering, and they will make vows to the LORD and perform them.