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 Et l'Eternel sera connu des Egyptiens, Et les Egyptiens connaîtront l'Eternel en ce jour-là; Ils feront des sacrifices et des offrandes, Ils feront des vœux à l'Eternel et les accompliront.