Short, because the service is small and the promises should be ones it can keep.
Nook is software you run. It is provided as is, and you are responsible for what you subscribe to.
You own what you write. Nook renders it and serves it, and claims no rights over it beyond what is needed to do that.
You are responsible for what you publish. Nook may withdraw publishing from an account that publishes unlawful material, and will say why. Withdrawing publishing does not remove your access to deletion or export.
Publishing is in a limited invitation period and is not covered by a service level agreement. Backups are taken and restores are rehearsed, but you should keep your own copy of anything you cannot afford to lose.
These terms may change. A change that affects ownership, deletion, or export will be announced before it takes effect.
Open an issue on GitHub. An email address for support arrives with the public release; until then this is the channel that is actually monitored, and pointing at one that is not would be worse than pointing at none.