I was impressed by that too. It is a very valuable skill, to be able to build a UI prototype that quickly. The author attributes it to Clojure and there is certainly truth in that, but given he also basically learned a new programming language in a day and got a decent rating on his solution makes me think it is not just the tool. For one, this is a smart and skillful programmer. But it also takes a playful mindset and training to be able to do this.