No. The focus module doesn't emit any event. You can see focus as a sub module of camera that contains the focus logic. The code above is overriding the default event handler of the focus module. The camera module mediates with mozCamera for everything regarding focus:

this.focus.onFacesDetected = this.onFacesDetected

The pattern above is the same you already see in the code when dealing with the mozCamera API directly. e.g:
  this.mozCamera.onShutter = this.onShutter;
  this.mozCamera.onPreviewStateChange = this.onPreviewStateChange;
  this.mozCamera.onRecorderStateChange = this.onRecorderStateChange;  
  this.mozCamera.onFacesDetected = this.focusOnLargestFace;