That is correct: They were broken off because of unbelief, but you stand by faith. Do not be arrogant, but be afraid.
The apostle proposes here a plausible objection, which might be urged against the divine conduct in casting off the Jewish nation (Rom 11:1): "Hath God cast away his people? Is the rejection total and final? Are they all abandoned to wrath and ruin, and that eternal?
Commenting on Romans 11:1-32
Well,.... To this the apostle answers, by approving and granting in, part what was said, that the unbelieving Jews were broken off and rejected, and that the Gentiles that believed in Christ were grafted in among the Jews that professed his name; but then he tacitly denies that it was for their sakes, and their account, they were broken off, but for their own incredulity...
Verse 20. Well. True. It is true they were broken off; but in order to show that there was no occasion for boasting, he adds that they were not rejected in order to admit others, but because of their unbelief, and that their fate should have a salutary impression on those who had no occasion for boasting, but who might be rejected for the same cause.