The Eastings in the geojson are Google mercator coordinates (EPSG:3857), but Northing is calculated froma virtual axis 12000000m North of the equator, positive southwards.
You could use a custom CRS like this:
+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=-12000000 +k=1.0 +units=m +nadgrids=@null +wktext +axis=esu +no_defs
But unfortunately, the special Google Mercator conversion does not work that way.
So you have to calculate new northing from 12000000 - old Northing, then do the standard Google Mercator reprojection from 3857 to 4326.