(Edit: No, this isn't the marketplace code, but the fact that they share a Github account is some evidence the marketplace could be reusing/extending CMS.gov code).
That's only the static information and FAQ site, not the marketplace.
From the readme:
>This project does not include any source code for the Federal Health Insurance Marketplace (the online systems located under www.healthcare.gov/marketplace.
Why not? Twitter, Facebook, Google, and Amazon all use a lot of Java. It's come a long way since when Microsoft shipped rollover buttons via Java applets in MS FrontPage.