From the article:

>Caching at many levels is of course still a part of our application, but so far we've not found any reason to cache a single MongoDB document retrieved by primary key in an external cache like memcached, a practice that is currently common for us when we use relational databases.

I'm not quite sure what they are actually caching as I can not find any valuable info in server headers at all, for example relating to varnish etc.