Merry Christmas!
Yep. That's the City of God. And that's a multicultural city. Augustine was advocating multiculturalism. So Augustine can't serve as an authority for the Williamson's pro-nationalist views.
Williamson recognizes this problem and tries to get out of it by spiritualizing this multiculturalism, viz., A closer reading shows, however, that the “citizens” are called “out” in a spiritual rather than a physical sense.
But this is just obfuscation.