The lads who were born to Supplanter, we discover later, bear the names of the tribes of the nation of Israel. The story of their births and their naming remind me of a Kipling "Just So" Story. You know, the ones that end with "and that's how the elephant got its trunk" and such like. And that's how the tribe of Exaltation got its name.
Whether these men were all sons of Supplanter or even existed at all is not particularly important. What signifies is that the story is later used to place the disparate tribes into a single superfamily. Just as patrilines are segments of clans and clans are segments of tribes, so the tribes are segments of a nation. All the groupings beyond the individual are predicated on the principle of patrilineal kinship. The tribes are probably related in some way, but the the accuracy of the genealogies is less important than the reliance on them as the predicate for a set of obligations and expectations associated with the kinship meme complex.
Now this concept of nation is quite different to what we nowadays think of as the modern nation-state. It is the family writ large. It is not necessarily a stable entity as loyalty to the progressively smaller segments and individual interests will tend to outweigh loyalty to the most distant and abstract of the groupings. Therefore, the concept of the nation must be reproduced continually and reinforced with myth. God is called upon to add the imprimatur of divine legitimacy to the idea.
Of course, some kindred groups are excluded from the nation, and the myths have to account for this. Otherwise, the progeny of the displaced first born sons would have to be included in the covenantal nation. I don't know why the mythmakers didn't just make their ancestors the first born. Perhaps it was deemed especially important for well being to leave the door open to exceptions to primogeniture or even to problematize the concept of primogeniture.
What we have in Genesis, I reckon, is propaganda to support the idea of a nation and conquest of a territory. Whether God promised anyone that the nation would rule the territory in question is beside the point. It came to pass that the territory was conquered and occupied by the putative descendants of Father of Multitudes. Hence, it was surely the will of God whether or not the nation knew what the will of God was in advance. The narrative, as it came to be written down, was for the purpose of reproducing the concept of the nation rather than offering what we would today consider history.