What Paul tells us here is consistent with the first promise made in Eden (Genesis 3:15) and the subsequent promises made to Abraham, Isaac, and Jacob. Every promise in the Bible which relates to salvation is connected to the covenant of grace.
As in English and Greek, the Hebrew word ‘seed’ can be a collective noun, a noun which is singular in form but refers to a group of more than one. Although the word is occasionally used to refer to a single individual, most of the promises made to Abraham use it in the collective sense. The Hebrew word on its own is not enough tell us whether it is used to refer to Abraham’s many descendants or to a single individual - Christ. So why does Paul make so much out of this one word? His intention is not to argue that that the word seed is singular in form and therefore should be understood of a single individual, but to guide us in how we should read this word which he knows could be singular or collective: the ultimate reference he tells us is to Christ.