It seems to me that the screening manager inherently has the discretion that the supervisor does not have -- and exercised it. What's the problem?

If I showed up at a checkpoint carrying the Hope Diamond (or some similarly valuable object) and requested a hand check, I would hope (pun intended!) that somebody would have the discretion to do exactly that! Some things are just too valuable to run through the x-ray machine -- or to put into checked baggage.

Bruce