I doubt WMF uses wgCacheDirectory at all. It's default is false and WMF has Memcached. It doesn't even affect WikiFarms since it's not set by default. You have to explicitly set it yourself.
It's absent from InitialiseSettings.php but in CommonSettings.php it's set as $wgCacheDirectory = '/tmp/mw-cache-' . $wmfVersionNumber;
* http://noc.wikimedia.org/conf/highlight.php?file=CommonSettings.php