1: Yes, it is open source. It's all just one html file, you can right-click and select View Page Source to see the source code. And all of the javascript and css that it references come from open source projects via their official content delivery networks.
2: No, I hadn't heard of that. It's using SimpleMDE instead. But thank you for pointing that out, I'll definitely look into it :) Quick question from my first glance at the demo, does it require JQuery?