So that can't be referring to the spiritual seed of Israel? If not, why not? Jacob prophesied that the seed of Israel (Ephraim) would become a fullness of the nations (goyim). That would not be realistic if he hadn't been referring to the spiritual seed. He gave the promises to Abraham, Isaac and himself to a half-Egyptian, which symbolizes this spiritual seed, the Israel of God, comprised of both the physical and spirtual seed of Israel.
And his father [Jacob] refused, and said, I know, my son, I know. He [Manasseh] also shall become a people, and he also shall be great, but truly his younger brother [Ephraim] shall be greater than he, and his seed shall become a fullness of gentiles. Genesis 48:19