Joshua came at that time, and cut off the Anakim from the hill-country, from Hebron, from Debir, from Anab, and from all the hill-country of Judah, and from all the hill-country of Israel: Joshua utterly destroyed them with their cities.
The words of God to Moses to which the writer alluded (Joshua 11:23) are probably those in Exodus 23:27-33 (cf. Deuteronomy 7:22). There God told Moses He would not drive all the Canaanites out of the land in one year but little by little. This is how the conquest of the land had advanced thus far and how it should have continued to its completion.