Those must be cached pages from dozens of servers on the backend.
There is no way in heck it's realtime queries.
I can tell when authors are in the WP backend on the server just by looking at the server load because it's crippling.
The wordpress bottleneck is not nginx vs apache vs whatever, it's the problem of loading hundreds of files for any kind of page render (even to just authenticate for ajax, etc.) and over a hundred db queries in many cases.
Cached or uncached is not relevant to the article.
They are talking about using nginx as a load balancer. It just proxies all requests off to the application servers that actually prepare the results. This article is really just about the fact that nginx can efficiently service a very impressive number of connections at once.
Those backend servers will be using caches of course though.
Many more dynamic pages than you would expect. We do hundreds of thousands of database queries/sec. We do some caching with Batcache - http://wordpress.org/extend/plugins/batcache/ but it's more to handle large spikes in traffic for single pages/blogs. CNN during the US elections, for example.
Wordpress is only reasonable fast if you use APC, memcached and the MySQL query cache. I never liked the caching plugins. If you use nginx just use the fastcgi_cache module and make cookies part of the cache key or omit the caching of pages where the login cookie is set.
APC+mysql cache (not just query but keycache too) is a must, but memcached is pointless unless you have a multi-server website (on a single server APC shared memory is way faster than memcached).
All logged out users should be served a completely cached static page that bypasses PHP entirely, it's way faster than even an opcode cache and way less load.
WP-Super-Cache can serve static pages to logged out users and makes this rather easy to setup. It's practically a must for wordpress.
I've did some experiments - and yes APC is a little bit faster (1-5ms per page rendering) when used as key-value store in comparision to memcached via unix socket. However I had always problems with cache fragmentation and a resulting slowdown. If you use Apache mod_fcgid the PHP processes are beeing shot down and started at random and APC is is not shared acress them. With nginx and PHP-FPM is is not so true anymore. I've had the impression memcached is more solid.
For static caching I use nginx with fastcgi_cache. This makes WP-Supercache obsolete. I bybass the cache for requests containing certain cookies, so that logged in users always receive fresh pages.
Another big Wordpress performance sink is locales. If you have a blog in a non-english language consider using a language-caching plugin. Wordpress uses a PHP gettext implementation that is quite slow. 20% speedup if you cache the generated locales.
But it's a shame to spend days or months on tuning Wordpress, what is essentially just a blog engine.
I don't know how the caching plugin in WP works, however the most difficult part of caching is always cache invalidation, which is the reason why Memcache is better than an in-memory cache, even for a single server, because then you have the flexibility of invalidating cache entries from a background process that does not run on that server.
It's also great to have the flexibility of adding more servers in case of a huge spike, even if you are running on only one server. Like on Heroku, where you can just increase the number of dynos in realtime. Doing that without a central cache that every server can access can break your MySQL because each new server will have a cold cache.
If you want a smart cache invalidation system for NGINX please have a look at this plugin http://bit.ly/nginxmanager
Pages are cached on NGINX for not logged in user, and when a new content is published all and only the pages that contains that content are deleted from the cached and regenerated at the next request.
It's actually not that different, and of course they make as much use of caching as possible.
So does my self-hosted wp blog, for that matter. If you are using WP Super Cache correctly, you can deal with significant amounts of traffic. I've seen peaks of hundreds of simultaneous users on my blog, with a server load < 1. The server is a modest aws small instance.
Every time an article is created, saved or published, wp uses hundreds of non-cachable queries.
Every time an article is published, it causes the cache to be deleted for not only that article but related pages, which means all those pages have to be rendered again.
For one author, that can be managed. Many authors, the cache is constantly being defeated.
I know they replace the database class on wp.com to support replication (I think they even released the code once) but I don't think they do extensive changes otherwise unless you've actually read otherwise.
There is no way in heck it's realtime queries.
I can tell when authors are in the WP backend on the server just by looking at the server load because it's crippling.
The wordpress bottleneck is not nginx vs apache vs whatever, it's the problem of loading hundreds of files for any kind of page render (even to just authenticate for ajax, etc.) and over a hundred db queries in many cases.
A cache-miss in WP is a terrible, terrible thing.