“Yahweh will be terrible

Against them.

He will shrivel

All the gods of the earth.

All the coasts,

All the islands,

Shall bow down,

Each in its place.”

Yahweh was all powerful.  He would make all the small idol gods of the earth shrivel up.  All the coasts and islands on earth would remain where they were, but they would bow down to Yahweh.  This seems like an addition here to finish up the destruction of Moab and Ammon.