In context, the body of believers in Christ are Abrahams descendants.
Paul said, "Has God cast away his people that he foreknew?" Look at his answer. He uses himself as an example,
And he shall set up an ensign for the nations, and shall assemble the outcasts ofIsrael, and gather together the dispersed of Judah from the four corners of the earth. Isa.11:12
I have chosen you out of the world, Jn.15:19
to Abraham.....the covenant.....was confirmed before of God in Christ Gal.3:16-17
Your point....your point, was how "seeds" refers to the nations of the world, but in that passage, Paul is using "seeds" only refering to Abrahams physical descendants, contrasting the two covenants. Hello?
I'm wide awake,
That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him Eph.1:10
I'm trying to get you to see that when God call Abraham to leave Ur, he was "calling him out" (ekklēsia) of the world.