What progress in salvation is to be seen in the reference to “my worshipers” from beyond the rivers of Cush?

During Israel’s slavery in Egypt, Pharaoh had been beaten down by the plagues, to the point where he urged Moses and Aaron to plead [עתר]with the Lord on behalf of his battered land (Exodus 8:8–9, Exodus 8:28–30; Exodus 9:28; Exodus 10:17–18). But now Zephaniah sees the day when the peoples beyond the rivers of Cush—south of Egypt—would have their own speech purified in order that they themselves might call upon the name of the Lord, and live as the Lord’s worshipers (עתרי).¹