I’m assuming they were able to somewhat isolate this fire. It can’t have been an ordinary fire if it burned almost 10 hours. Where did it get the oxygen? It must have been a chemical fire where the oxidant was built in. I don’t imagine too many details will be forthcoming.
Subs carry VERY large amounts of high pressure air and high pressure oxygen. If they had those systems charged at this point in their shipyard work, that could be the answer to your question. Alternatively, they might have had a very large hole in the hull for maintenance, which would give them an unlimited supply of fresh air. I suspect that someone on FR knows the actual answer.