Isaiah calls them inhabitants of this coastland because Judah wanted to turn from God to trust in human power instead. If they wanted to trust in Egypt instead of God, they would be mere inhabitants of the coast instead of a holy nation in the holy land.
6 And the inhabitants of this coastland will say in that day, ‘Behold, this is what has happened to those in whom we hoped and to whom we fled for help to be delivered from the king of Assyria! And we, how shall we escape?’”