There are two options. If Goldberg is correct, then J-Street calling themselves "pro-Israel" is an absurdity - you cannot be pro-Israel when almost all Israelis disagree.
If Beinart is correct, then I can say with more certainty that I am pro-Palestinian.
After all, according to Beinart, it is up to individuals to define whether they are pro- or anti-something, and objective reality is not relevant.
I support the right of Palestinian Arabs to live in peace and security in any Arab country without discrimination. I support equal rights for Arab citizens of Israel. I support helping the economy of the territories. I am very opposed to Arab discrimination against Palestinians. I condemn how Lebanon and Egypt and Jordan and Gulf countries treat Palestinians as second class citizens.
Therefore, I am pro-Palestinian, by Peter Beinart's definition..
And my pro-Palestinian credentials actually outweigh J-Street's pro-Israel credentials, because I have lots of examples of Palestinians who (among themselves) agree with everything I just wrote, while J-Street will have to dig around the extreme Left of Israeli politics to find those who agree with them concerning Iran.
The fundamental question is whether being "pro-" something is objective or self-defining.
By objective standards, J-Street cannot claim to be pro-Israel if actual Israelis who have to live with the consequences of J-Streets positions consistently disagree with them group.
By Peter Beinart's standards, if someone wants to claim to be pro-Israel then they are. Presto!