The journal is stored locally on your device, to ensure privacy. With iCloud enabled for the app, the journal is also synced automatically across all of your devices. Other cloud document storage options that may be installed on your device are not used for storing or syncing the journal..
The Document Storage option is available in the app in order for you to be able to save a copy of a journal on other document storage systems (backup), and merge a copy of a journal back to the locally stored journal (restore). You can use the Manage Journal feature from the Action menu in the Journal screen to export or import journals with cloud services.
If you change the Document Storage option to anything other than iCloud Drive in the Settings app, then syncing the journal between devices will be disabled.