Before we all over-fulminate about the debris being found, let's all remember that it was found, and moreover it was found in the course of an inspection made specifically to ensure that any debris would be found.
Having found it, it would be equally a matter of good routine and practice to report it with a view to pre-delivery prevention of debris in fuel tanks in the future. (Boeing MEDA process, for those who know, as copied throughout the industry).
The inspection was/is a capture point designed to make sure that no debris is present, and as such it worked perfectly.
End of story. A more sensational story would have been "Debris not found in B737 fuel tanks before delivery".
Boring, eh?