BEV thermal runaway fires are certainly not a solved problem. I'm not sure they are as common as some critics suggest, but they are difficult when they happen.
Even without BEVs a fire in a car park can easily result in the whole thing burning to the ground if you're not careful. It happened at Luton a few years ago, and I think in the end they worked out it was a diesel that started it.
You're right though, if every car is a BEV of unknown battery health left for a week+ packed bumper to bumper in a baking hot car park in the middle of summer, that would definitely be considerably worse
> I'm not sure they are as common as some critics suggest, but they are difficult when they happen.
Indeed, and the usual solution that firefighters choose is to dump copious amounts of water onto the burning vehicle and then drown the carcass into even more water. The biggest issue is that it takes time for the firefighters to arrive - about 10 minutes in Germany by law - and so a fire has quite the time to progress.