Under the UNIX way of understanding the universe, the name of a file belongs to the directory that the file is in, not the file itself. After all, a file may have many names.
The logical conclusion of this is that the "secure delete" setting of the file itself should not affect whether the filename is securely deleted; rather, it is the setting of the +S flag on the directory containing the name that should matter.
Attributes of the file refer to the contents of the file. Attributes of the directory refer to the contents of the directory (the filenames in that directory).