The title is wrong. Like you said in article, it's not a WordPress server but only a nginx serving static content.
Only 2 persons hit PHP on your Pi, you when you edit posts and the first visitor for cache generation.
Only 2 persons hit PHP on your Pi, you when you edit posts and the first visitor for cache generation.
(Imagine 5,000 threads all deciding they want exactly the same data at exactly the same time, then trying to write it to exactly the same location. Now imagine 50,000 more threads trying to do exactly the same thing because of the delays caused by the first set. Now imagine your web site is down and your mobile phone is ringing)
The bigger problem is when your entire cache is cold (ex. memcached was restarted) and there's a ton of traffic to lots of different content. A single piece of content should not be that crippling, unless it's stupidly-slow to render and cache.