The issue isn't so much an explosive risk it's more a projectile risk. Packs are generally made of the same round cells we see in USB battery packs and those mechanically are very much like a drinks can with a pressed can body and base and then the top crimped on.he was telling us about Electric cars/vans and how there are a couple of garages that have an authorised Lithium battery pack repair area that are built like bomb shelters. . . where the specially trained guys have to wear full on explosive proof gear to disassemble and make safe damaged battery packs. . and perform battery pack repair.
There is a pressure vent to help prevent issues under normal conditions but in a large pack even when discharged (and a damaged pack may not be able to be) there is still substantial energy. If enough current flows to start a fire the over pressure vents can't cope and the tops burst off the can leaving the body very much like a firework rocket motor.
It's why I'm very wary of DIY power walls etc, if you have enough cells in one place you really need to consider mechanical containment/moderation so that you minimise the risk of cells ejecting, as burning rocket motors going everywhere in a fire really makes it hard to deal with.