It doesn't even look like its setup correctly... the repl buffer name should read slime-repl clojure not inferior-lisp. inferior-lisp is what you get when you haven't properly connected to swank.
Such a horrible presentation :( You can't see anything. He's actually using a pretty awesome screen recording tool that can zoom in into any part of the screen. It takes about 5 min to fix it in postproduction.
What's even more confusing is why HN would upvote this over 17 times. Probably only because the title has Clojure in it's name :(
What a useless link.