There're a lot of sour grapes in this thread about performance and made up metrics, but let's take them at their word:

For a site as seemingly simple as Medium, it may be surprising how much complexity is behind the scenes. It’s just a blog, right? You could probably knock something out using Rails in a couple of days. :)

This hints that there is a lot more functionality being put up behind the scenes in terms of datamining and analysis and whatnot than is apparent from the simple "display an article", which indeed is a trivial Sinatra or Express (not even Rails!) app.

If you're not the customer, etc. etc.