Their source code isn't located there anymore. When they took down the site, they deleted everything from their Github page as well. If you want to use it, you'll need to find a user who saved a copy of all the source code files before they were removed and request that they send you the files.
Not exactly. Their source code is still there, but you'll probably need git (rather than just downloading a zip) to get it.
This should work:
git clone https://github.com/brainwallet/brainwallet.github.io.git
cd brainwallet.github.io
git checkout f7679dd0
f7679dd0 is the last commit before the site was closed, it looks like everything is still there. Please ONLY use this to recover an old brain wallet, don't make new wallets with this since we all know it's broken.