As someone who has actually put out numerous car fires I would say that this estimate wildly depends on the time at which the fire occurs for ICE engines. These days most ICE based cars are pretty easy to deal with and if you arrive early enough to the scene you may even be able to put the fire out with as little as 1 11L Compressed Air Foam Backpack. In fact good car models can keep the spread of the fire contained to just the bonnet for quite a long time. I suspect with electric cars the challenge is that the flammable material is directly beneath the passengers and adding water may actually make the situation worse initially. I highly do8ubt a single fire engine will be able to carry enough water to combat the fire. At least electric cars are better than CNG cars which are a one way ticket to permanent retirement for firefighters.
Dumb question ... but do "inflatable swimmingpools" exist for firefighters? Something that you can roll up compactly but unrolls around e.g. a car and forms a more or less watertight seal so you can literally drown the battery in water?
An inflatable swimming pool relies on the watertight bottom to hold shape. Without a sealed bottom you're building a flood barrier. You need something that both molds to the shape of the surface, and is heavy enough to provide the downward force to prevent water from going under it. Inflatable flood barriers exist and are filled with water.
I guess you wouldn't really need it to be extremelly watertight, just tight enough that the hose puts more water into it. Seems like it would pay for itself with 30k gallons needed