For me it's the fact that he's still coding, putting thought into dev tools and his dev setup (less is more in his case).

Nix is hardly simple

On the face of it not, no. But when you consider the immense power and versatility of it (you can do anything from setting up simple virtual envs to automatically deploying a multi-container setup on AWS), it starts to become a lot simpler than the sum of all the other tools that it replaces. Nix is a single principled idea, but can be used in so many different contexts.