Not sure how anyone could ever reach such a conclusion when reading the text.
Romans 11
13I am speaking to you Gentiles. Inasmuch as I am the apostle to the Gentiles, I magnify my ministry
14in the hope that I may provoke my own people to jealousy and save some of them.
Paul is referring to the fulfillment of prophecy.
Deuteronomy 32
21They have made me jealous with what is no god; they have provoked me to anger with their idols. So I will make them jealous with those who are no people; I will provoke them to anger with a foolish nation.
So, yes, these Gentiles had believed and so they were included/grafted in to His people. Just as the verses that followed state.
Romans 11
17Now if some branches have been broken off, and you, a wild olive shoot, have been grafted in among the others to share in the nourishment of the olive root,
18do not boast over those branches. If you do, remember this: You do not support the root, but the root supports you.
19You will say then, “Branches were broken off so that I could be grafted in.”
20That is correct: They were broken off because of unbelief, but you stand by faith. Do not be arrogant, but be afraid.
21For if God did not spare the natural branches, He will certainly not
g spare you either.
22Take notice, therefore, of the kindness and severity of God: severity to those who fell, but kindness to you, if you continue in His kindness. Otherwise you also will be cut off.
23And if they do not persist in unbelief, they will be grafted in, for God is able to graft them in again.
24For if you were cut from a wild olive tree, and contrary to nature were grafted into one that is cultivated, how much more readily will these, the natural branches, be grafted into their own olive tree!
So, clearly, they do have faith, and have thereby become God's people He chose in the stead of the unbelieving Jews. There is no other legitimate reading of the text, which explicitly teaches these things.