Two parts to my question.
Seeking safety...
To store? I never heard anything about storing but I guess it doesn't hurt. Those bags are more for charging when the battery is most likely to go off. They prevent fires by containing the fireball so it doesn't spread while relieving the gas pressure of the explosion (if you just contain the pressure then you end up with a shrapnel-producing bomb). This is also why you leave a crack in the lid and do not seal it shut if you are charging in an ammo box.
Even if the steel in the ammo box can burn, surely it's more difficult to set off than everything else around it? And I don't think we're talking about something like thermite here. The steel shouldn't ignite.
How do these help prevent fires?
The batteries will catch fire, but the container will prevent further spreading of the fire.
With the oxidizer a component of the Lipo battery, would storage in an all-metal ammo can be a good idea.
You would probably want something that prevents heat transfer. A metal can will still get hot on the outside. You'd probably want something that is made from insulation.
If you're keeping the batteries long term, drain them to 50% and keep them at their nominal temperature.
