How so?
Basically, the idea is that Yi is actually a Haskell library, and your "config file" is really a Haskell program that uses this library to build an editor.
This is similar to how many suckless tools are written. If you want to configure them, edit the source: It's designed to be approachable and easy to modify.
This style of configuration is extremely nice IMHO. It gives you complete control over everything in one of the cleanest, most expressive languages around. It also means that you don't need to maintain a separate configuration language, and people that use your software are already taking steps towards becoming contributors.