How is this an API quirk? It's normal and expected behaviour with file-handles in Unix.

Isn't it entirely the expected and long-standing behaviour of filehandles? A user-space programme can acquire a file-handle for something, the object itself can be removed from any indexes that make it visible (e.g., where the file-handle references a file, the programme itself or another can delete the file-name entr{y,ies} from the filesystem), and the programme could if it wishes make the referred to object visible again (adding a file-name in the filesystem again for the file-handle).

This is entirely normal, expected behaviour with file-handles, the referenced object, and whatever generally queryable indices there may be of such objects (e.g. filesystems).

I think, at best, the author means this *normal behaviour* is - for some reason - not desired for namespaces. And the reason why namespaces are special, compared to other objects, should then have been explained.