The domain layer is separated from the other layers and has no dependencies on Zf_Controller. Adding the domain directory to your include path allows you to load and access Model classes from anywhere within your project.

His code examples show an IndexController extending the Zf_Controller and how a Model can be called from inside it through a getInstance (as long as the model directory is in your path). You can take a look at part one and part two for more information on this alternative component.