Answer: the meaning of the promise is determined by the Biblical use of those phrases. Other passages describe groups of people using precisely those words. For example:
Joshua 11:4 - They [a coalition of local kings and their armies] came out with all their troops and a large number of horses and chariots—a huge army, as numerous as the sand on the seashore.
Judges 7:12 - The Midianites, the Amalekites and all the other eastern peoples had settled in the valley, thick as locusts. Their camels could no more be counted than the sand on the seashore
Samuel 17:11 - "So I advise you: Let all
Israel/country-region>, from Dan to Beersheba/place>/city>—as numerous as the sand on the seashore—be gathered to you, with you yourself leading them into battle.
Nachum 3:15 - You have increased the number of your merchants till they are more than the stars of the sky, but like locusts they strip the land and then fly away.
Thus it is incorrect to say that the promises have not been, or could not be fulfilled.