It's spelled out pretty directly: "if you delete something, save your progress, and close your project, you will never be able to restore what was deleted"
What you said may apply to both "non-destructive" and "destructive" editors just as well. A better example would be "if you add reverb to your track, save your progress and close your project, you won't be able to unreverb it". With "non-destructive" editor, all you did was to add a flag that your track should have a reverb added to it, so you can easily disable or adjust parameters of your reverb afterwards without having to rely on undo.
