While QF is responsible for their supply chain, there should be processes in place (such as the standard across the food industry of First In, First Out) on Qantas' end to prevent stock being stored too long. If the problem was on Qantas' end, then there would be more issues then just a handful of packets as the entire stock at the originating port would be compromised.
If this was something like a meal which is fully within the control of Qantas (such as something made by Q Catering), then yes, it is QF's problem. However all QF would have done in this case is take delivery of the stock, open up the boxes, put a packet of the nuts into a Snack on Q bag and send it to the aircraft for delivery to the passenger. It's a premade, prepackaged, sealed item. Qantas has no reason to assume that it does not contain what the manufacturer has told them it does.