The flightdeck crew could be on 100% oxygen supplied at positive pressure but do passenger oxygen systems have this function or is it mixed with cabin air?
If your question is in relation to this flight then it's completely irrelevant, there was no smoke, no fumes in the flight deck and the crew were not on oxygen.
If it's a general question then you should really ask it in tech log, however you are correct, the passenger oxygen supply is mixed with air from the cabin, it is only designed to help with breathing in the event of a decompression and it has absolutely no value if there is smoke in the cabin, which I stress there wasn't in this case.