I think they were trying to emphasize that their strength in "readability", hence Medium.
I watched the video, and immediately realized I would never want to program in that kind of environment. The "literate programming" would get in my way.
It would be like having someone follow me all the time and translate my words in realtime right next to me whenever I speak something. (Actually it's more like me saying something in English AND having to translate after every sentence) It's super distracting because I can't focus on what I'm saying because I lose my train of thought.
I would say its closer to the reason we include emoticons in text messages. Text alone loses something from the original intent, so we add emoticons as a way to bring some of that back. Plain code as well cannot express softer yet still crucial aspects of software design like "intent" or "audience" of the subsequent code.
Emacs is better ;) But the comparison is a bit disingenuous. Medium is a blogging platform, VIM is an extensible text editor.