I tried to do something similar once, I even added cars of my own, but not nearly with this diversity in terms of buildings and so many streets.
But I am confused about something: from the top wireframe view at least, it seems that this map are just a bunch of mostly shallow cubic buildings, so how come he got up to that many nodes?
Maybe the screenshot is so low-res that I am missing the actual details there, so is there a higher resolution screenshot at least?
(I don't have Facebook, and lately there has been even stronger reasons to not ever have one)
I would say that even before thinking into mesh conversion (although for some things it may indeed be the best course of action), since all the map is using solid BSP, many of those could be turned into semi-solids lowering that node count.