My understanding is that the outboard leading edge flaps move during Flaps 1 to 5 (and 5 to 1) and their influence on the fuel stream could cause fuel to contact the trailing edge of the wing.
The jettison nozzles are outboard of the low speed ailerons near the wingtips and well away from the trailing edge flaps. Of course, the t.e. flaps are HUGE and must also influence the airflow in that area.