One could argue that the promise that Abraham would be the father of a great nation was fulfilled while the Israelites multiplied in Eypt:
Deut 26:5 you shall make this response before the LORD your God: "A wandering Aramean was my ancestor; he went down into Egypt and lived there as an alien, few in number, and there he became a great nation, mighty and populous.
In fact, when the Israelites sinned, God tells Moses that he and his descendants would take their place:
Ex 32:10 Now let me alone, so that my wrath may burn hot against them and I may consume them; and of you I will make a great nation."
The contrast is clear. Israel was a "great nation" and God was rather angry and wanted to replace them with Moses and his descendants.
In any case, one could argue that Israel fulfilled the promise of being a great nation while they were in Egypt.
Additionally, 1st century Jews understood the promises of Abraham to include promises to the entire earth. They understood the promises to Abraham to include an inheritance of and blessings on the entire world. Abraham was to be the father of many nations (Gen 17), not just one. They believed the promises were not limited in scope to an ethnic group to occupy a small tract of land.