But it is specified. God told Israelites to purge Canaan of human life. Ethnically cleanse the place.
If nobody exists in the country aside from your brothers, who else is going to be your neighbor ?
Also God said dont allow foreigners in and dont mix with them, dont worship their gods and so on.
YES God doesnt specify in that specific paragraph, because you need to take the whole Bible into context.
God is a being where years are but a moment to Him. You cannot expect Him to keep repeating Himself, because for Him, He just told you. (well He told your ancestors to tell you)