Issue tracker on GitHub... Hang on: NO SOURCE? Is this open source?

Imagine my delight when I see that the link to a "Github issue tracker" on the home page. So I got to Github and check it out... And then I go to check out the code. What the???? No code? What is going on here? Is this open source? or are you merely using Github as a free issue tracker for your (boo) Non-open source project?