Its simple actually. Let me spelt out the argument slowly.
Abraham was given many blessings: of land, of righteousness without works, of many nations, etc throughout scripture, for example in Genesis 12, Genesis 15, Genesis 17, and Genesis 22.
One particular blessing that he gotten, righteousness by faith alone (Genesis 15:6), was given specifically before God commanded him to physically circumcise himself in Genesis 17.
His sole child of promise, Isaac, cannot claim to have that since he was circumcised at birth, nor can Isaac's son Jacob as well.
That is why, if you read Galatians 3 carefully, we got the blessing of Abraham (singular) (Gal 3:14), because being in the Body of Christ, we are also the child of promise.
That singular blessing was the main one given before circumcision, so we gentiles in the Body of Christ can legitimately, in the eyes of God, be justified by faith alone without works (Romans 4:5).
But the other blessings of Abraham, that was repeated to Isaac, repeated to Jacob, are not for us to claim, the physical land promise etc. Israel as a nation was given many other promises by God because of the Law, because of the Old Covenant, and, in the future millennial reign, the New Covenant.
If you anyhow think you are also the seed of Jacob, then you will also think there is a physical land waiting for you to inherit on Earth as well, or that you will be also under the New Covenant which is scripturally incorrect (Hebrews 8:8 Romans 9:4)
Hope this presentation is clear why being Abraham's seed is not equivalent to being Jacob's seed, and why that distinction is important.