I used to be a major contributor and follower of Atom. People commonly asked why Atom couldn't run in the browser. The answer was simple, node and a browser are totally different things. Imagine a browser opening files, starting processes, running arbitrary C-code, etc.
What everyone else has been doing is embedding Monaco (the barebones text editor component) into their shell. The shell implements things like searches, key bindings, a file explorer tree view, top menu, fake-console etc.
What the creator is saying is that he got the actual VSCode project running in the browser. This means that the vast plugin ecosystem will run inside the browser. I suppose he'd have replaced all the nodejs dependencies with browser versions; for example an "fs" module that keeps files in-memory (or maybe using indexeddb) etc.
Further down that tweet, he says that the code is very easy to get started with. That's been my experience looking at it briefly; VSCode just seems like it was written by a team with deep expertise in IDE-making and PLs.
