Their screenshot comparison is inaccurate. It didn't look right to me, so I duplicated their screenshot on my own Mac at 2560x1600. My Mac Portal screenshot matches their PC screenshot in sharpness. For some reason it looks like the Mac version was not running at 2560x1600 (or even close) in their screenshot.

Here is the Mac screenshot I made: http://bayimg.com/image/eamojaack.jpg

Their Mac one: http://images.anandtech.com/doci/3726/Portal2Mac.png

Edit: thought_alarm noted that there was just an update that fixes "screen fuzziness caused by color correction operation". That's probably what was going on.