It's difficult to tell but they have done quite extensive customization on that site.
For the notes, it appears they are just questions. The "Post a note" must be a custom plugin that adds it as a question to the database, maybe with a particular tag or category. Then the questions page is removing the questions with that tag/category, and the notes page shows only that tag/category.
If you wanted to do something like that yourself, the easiest way would be to add a "notes" category, then link to the ask page for that category (example.com/ask?cat=3). You can modify the theme to highlight notes in the question list.