If you need another example, look at https://dev.to/ for a fantastic blog UX, and it's open-source so Medium can just copy it for free and end up with a better product.
While WordPress still dominates large areas of the web, they have a decentralized model vs medium. It's like comparing scaling web forums to reddit ("why do they have so many outages? All they do is store text!"), but reddit obviously has a way higher load than any single forum. I'd wager the same is true for medium.
To continue on with the metaphor, medium, like reddit, is solving a different problem than the wordpress/squarespace model: you don't have to roll your own. You get a built in audience due to centralization and the cache of the site (For some reason I associate a medium link as being more legit than seeing a WordPress hosting site. Strange, but better branding I guess.)
Scale is differently, but again there are plenty of sites with far more complex usage and data that took much less to create and run. Stackoverflow being that example.