Galilee of the nations is not a detour. It is the map of the promise.
Isaiah saw a great light in Zebulun and Naphtali, and a servant who would not stop until the coastlands waited for His law.
Matthew quotes Isaiah 9 when Jesus settles in Capernaum. The light starts in despised territory and runs outward. Chapter 28 is that light finishing its road.
Paul later says he is sent as a light to the Gentiles. That is the same Isaiah program in the same generation.
The nations walk in a light that has already risen.