That's not true. And to the extent that they did believe that, that's not the same thing as saving faith. They are natural branches, on account of being Israelites, according to the flesh, and their lack of faith, though removing them from the blessings of God's promises to Abraham, does not change the fact that they are the natural branches and the Gentiles are the unnatural ones. All through Romans 9-11 Paul refers to Israel, and it's always ethnic Israel, and up until that point in chapter 11 when he reveals God's plan of all Israel ultimate coming to faith, he talks about their lack of faith. Even while they lack faith, it is still the case that to them (Israel views as a group) belong the covenants, the promises, the patriarchs, the adoption as sones, the giving of the Law, the glory, the worship, and the coming of the Messiah according to the flesh (9:3-4).
The cutting off of the natural branches is not saved Israelites who then changed their minds and lost their salvation, it's these same unsaved Israelites who never had salvation that Paul has been discussing all through Romans 9-11. The ones who sought righteousness by the Law, and thus never attained it.
And then, again, when Paul talks about natural branches who had been cut off getting grafted back on, he's not saying that the very same individual Israelites who disbelieved will later come to faith, but that Israelites in general will. My interpretation of this is that this coming salvation of all Israel is still yet future. If I'm right about that, then it can't be the same individuals getting grafted back on who were cut off in the first century, it must be a completely different generation of Israel.