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