I think puntosaurus's quotes from the AAIB are key here. The tanks weren't damaged, and the wreckage was nose down. The front bottom of the supply tanks are closed, and so what was found in there should have been there on impact, give or take a tiny bit of splashing through the overspill holes.
If the aircraft had come to rest tail down, then it might have been ambiguous, as some of that 76kg of fuel could perhaps then have made its way through the overspill holes from the main tank into the supply tanks. The investigators have a lucky break that it didn't.