It's not so much (or just) walled gardens, its the hiding of the more fundamental layers of the system from the user.

A user doesn't navigate through the file system to a folder where they've saved a bunch of notes in discrete text files of some format or another, they start up a notekeeping app and use its interface to open their notes. Where are these notes saved? That is either hard to find in the bowels of the settings of the app, or entirely hidden and they're saved in a database accessible only to that specific app.