There could be a buffer between the tank and wing surface, but the low pressure that forms above the wing would suck that fuel out anyways. Trust me, there have been Cessna pilots that left the fuel caps off after refuelling, the low pressure once the wing is generating lift will suck the tank dry.
I think in this case there was so much fuel onboard that the crew didn't have to rush to get on the ground, even if they were aware of the fuel leak they probably had more than enough fuel to continue dumping more of it with no concern.
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