The "Noteboard" is just the current view. If you're in the "Home" view that is a collection of all your cards, so exporting from there exports all of your cards in one go. We also have an API[1] that allows you to easily fetch the raw data (as JSON) directly if you want to export Supernotes-specific metadata that is not encapsulated in the Markdown.

[1] https://api.supernotes.app/docs/swagger