‘Thus says the Lord Yahweh, “This shall be the border by which you will divide the land for inheritance according to the twelve tribes of Israel. Joseph shall have portions (i.e. two portions). And you will inherit it, one as well as another, concerning which I lifted up my hand to give it to your fathers, and this land will fall to you for an inheritance.”
God was here reiterating to the exiles that He would restore what they had lost, and He then outlined in the following verses the land that was to be theirs for the taking. The land was to be split equally between the tribes ‘one as well as another'. As often Joseph was to be split into Ephraim and Manasseh for this purpose, and Levi had no portion (Ezekiel 44:28 - apart of course from the portion mentioned in Ezekiel 44:4-5). Thus the number twelve was maintained. Yahweh had sworn (lifted up His hand) that it would be theirs, and theirs it would be. It was their inheritance.
The fact that the land was to be split relatively equally, and to some extent without regard to previous tribal portions, or tribal numbers, is also an indication that it is not to be taken literally. Add to this that the tribes are split into a group of seven, indicating the divine perfection of the event, and a group of five, indicating conformance with the covenant, and the case is even more certain.
Many Israelites would in fact already be in the land having escaped the deportations. There were those in Galilee outside the range of the Samaritan importations; possibly those in Benjamin which was already in Babylonian hands prior to the final invasion, and may have escaped deportation; and some in areas to the south in the Negeb and the Shephelah which would possibly not been quite so affected (confirmed by Nehemiah 11:25-35). The King of Babylon was angry with those who had shown fierce resistance and probably finally saw the Judah and Jerusalem that he deported as a limited area. The idea is really that all twelve tribes will be represented in the land and be settled there, within the covenant, and within God's divinely perfect plan. All with a fair share of the land.