EDIT: Ha. My apologies as I just realized my stupidity - I was browsing on mobile and it turns out out I jumped into the 'What is Dark' post [1] and never finished the original! The mailing list links are in that one - I subscribed. This is a lot more interesting, cheers :)
The section that describes how it does it in 50ms is called "#deployless". That describes the mechanics of the deployment. How we made deployless possible is addressed for the rest of the post after that.
(The mailing list thing is weird, I only see that link at the bottom. Can you point me at the ones you see in the text?)
[1] https://medium.com/darklang/the-design-of-dark-59f5d38e52d2