My own opinion: the navigation system seems overcomplicated. I don't see this beating workflowy as it stands.
The navigation is separated in three broader levels. 1. The first level is everything that is relevant in the moment (Inbox, Next, Away). 2. The second level is to organize everything Idea, Project or Archive. From time perspective mostly related to the next weeks. 3. And the third level is to review your hole system.
After a short time it will feel like walking through different rooms and levels in a building.