Or HaChaim on Numbers 33:52/אור החיים על במדבר ל״ג:נ״ב

1 והורשתם את כל יושבי הארץ, "You are to drive out all the inhabitants of the land, etc." Even though the Torah says in Deut. 20, 16 that: "you must not allow a single soul (of these seven nations) to survive," in this instance the Torah does not speak of the seven Canaanite nations but about others who lived amongst them. This is the reason the Torah chose its words carefully, i.e. "all the ones who dwell in the land," that the Israelites were to drive out even those people who lived there who were not members of the seven nations.

2Alternatively, the Torah refers again to a nation which the Israelites do not possess the strength to kill. At least they should ensure that they would leave the land and not remain in it.