Hebron was explicitly mentioned as one of the cities that the twelve spies visited when the people of Israel initially came to the land of Canaan (Numbers 13:1–2, Numbers 13:22). In that account, we are told that the descendants of Anak lived in Hebron. Later we are also told that the descendants of Anak come from the Nephilim (Numbers 13:32–33). The Nephilim were tall and strong, so much so that they seemed like giants in the sight of Israel. The fact that the king of Hebron is joining the alliance means that Israel will have to fight against an army of men bigger and stronger than they are.
3 So Adoni-zedek king of Jerusalem sent to Hoham king of Hebron, to Piram king of Jarmuth, to Japhia king of Lachish, and to Debir king of Eglon, saying,