You're mistaken on that. Electrolysis is going to be far too slow to actually amount to any substantial amount of hydrogen and even if that wasn't the case burning all of that hydrogen would release only as much energy as it already took away from the reaction. Burning 100% of the hydrogen produced would still put you at a net 0 change in energy. For every joule worth of hydrogen that escaped without burning that's a joule of energy that you no longer have to deal with. Also it's a shorted out battery pack we're talking about. The voltage potential isn't going to be anywhere near as high as it would normally and if it was they you wouldn't have a flaming battery pack in the first place.

As to the notion that there's lithium metal in the batteries that could react with the water, you're thinking of non-rechargeable lithium batteries. Lithium ion cells use a lithium compound, not just pure elemental lithium. The formulation might be something like LiFePO4 but the point here is that it's not just lithium metal sitting around inside of it.

Also Tesla has an emergency response guide that includes instructions on fighting a battery fire. It's on page 22 (23 of the PDF) and it makes it clear that you want to douse the thing in as much water as possible and keep dousing it until it's totally gone. Tons of water is exactly what's called for when fighting these kind of fires.

https://www.tesla.com/sites/default/files/downloads/2016_Mod...