I would think the answer should be found in the passage about the center tank pumps:
Note: Fuel pump LOW PRESSURE lights may flicker when tank quantity is low and the airplane is in a climb, descent, or on the ground with a nose-down attitude.
Some time ago when picking up an aircraft from maintenance it was still in the hangar and the engineers were finishing up when we got there. Apparently they had at some point during the maintenance emptied the fuel load of the wings into the center tank, and were now transferring it back using the center pumps. They were unable to pump the last barely 500 kg from the center tank back to the wings (the pumps where not taking any fuel and had low pressure lights illuminated).
Hence, my personal theory is that the pumps probably run dry with less than 453 kg on ground in certain situations when the aircraft is parked on a downslope or maybe even just when empty and the gear struts extended. Probably for that reason Boeing decided to mandate that they at all times must be kept off if below this value.
With any automatic system of any kind, it's normally good practice to set the relevant manual switch to the agreeing position. Automatic systems can fail and the auto shut off feature of the fuel pump should be regarded as an extra protection, in case someone negligently forgets to turn the pumps off. It is therefore also still required that the flight deck is manned as long as the center tank fuel pumps are operating.
P.s.
And not all NG's have the nitrogen generation system, probably far from since it wasn't introduced until 2006.