Right after the terrifying vision of Gehinnom and the four kingdoms, the Lord sets a covenant. And Targum Pseudo-Jonathan on Genesis 15:18 spells out the promise with an emphasis the Hebrew leaves implicit.
On that day the Lord ordained a covenant with Abraham, that He would not judge therein his sons, but would deliver them from the kingdom.
Notice the order. First: I will not judge your children the way the wicked are judged in that furnace you just saw. Second: I will rescue them out of whichever kingdom of the four happens to be holding them at the time. Third: and by the way, here is the land — from the Nile of Egypt to the great river, the Pherath, the Euphrates.
The Targum reads the land-promise as the end of a rescue plan, not the start of a real-estate contract. The boundaries matter because they mark the safe country at the far end of the fire. Abraham is told in one verse that his children will be spared the worst punishment, delivered from the empire that enslaves them, and given a wide homeland to breathe in.
The Maggid hears the Lord's whole promise as one motion: no condemnation, no captivity, your own land (Genesis 15:18). After a vision that long and that dark, the covenant is exactly what Abraham needs — not an explanation of the furnace, but a word about where his children end up once the furnace is behind them.