If the nut is over-tightened, it remains in one position. The cable can move in/out, but to be able to move the cap more freely, the spin nut needs to operate as intended.
The screws around the filler neck and the clamps keep things pretty well in place. Early 64.5 cars didn't use a retainer cable and the filler neck stayed in place just fine.
I looked in the 64-65 Mustang Chassis Assembly Manual (AM0005 pg39), the "1965 Comet, Falcon, Fairlane and Mustang Shop Manual" and 1966, Ford Car Parts, 60-68 Illustrated Section 90, p11, Text 90 p3, and found only one mention of that nut, 380331-S8, a torque spec of 30-43 inch pounds. There is no mention of spin in any of the normally available documents.
Is there a TSB?
I know that nut/cable arrangement is a pain to contend with or adjust, but where does any reference to "spin" exist?
(The factory used a red foam gasket/sealer from the captive washer to the body.)
Jim