@Patty
If whatever started the fire had it's own oxidiser then depressurisation is not going to prevent its continued combustion.
It might help with other cargo that has been set alight.
I'm not certain that lithium batteries contain enough oxidiser to continue burning with little external oxygen, but the real problem is that they tend to be shipped at least partially charged because if the voltage is allowed to drop too low then the battery becomes damaged and is of little use. Hence there's plenty of stored energy to generate heat. Even modern NiMH batteries are going this way, the hybrid versions have a much lower self discharge rate so are now charged after manufacture and are advertised as "ready-to-use".