It still is a single single-core server, dang references it frequently when there's unusually high traffic [0]. And the language you're referring to is Arc [1]. They do have caching for not-logged-in users, historically done through nginx [2]. From other comments in this thread, it sounds like they just temporarily put Cloudflare in front of that single server to block a DDoS.
[0] https://news.ycombinator.com/item?id=38310213
[1] https://arclanguage.github.io/
[2] https://news.ycombinator.com/item?id=26473226