I may not grasp every thing that all the different eschatoloty groups believe but this statement seems odd to me with my limited knowledge of postmillennialism. Maybe I'm missing something. If the author defines Jews as an Biblical Ethnic group then I would see this as odd since most Postmillenialist that I know are calvinists and they don't see Jews as a Biblical ethnic group embraceing Jesus before the future second coming, assumeing they define the second comming as in the future. Many classic and some neo-postmils see a significant conversion within ethnic Israel during the future spread of the gospel throughout the world.
E.g., see The Puritan Hope by Iain H. Murray or The Restoration of the Jews by David Brown.