Technology

Diesel is of course flammable, it's used in internal combustion engines after also but as far as flammable liquids go it's not very easy to ignite, you need higher pressures and/or heat to get it to burn, under normal conditions you'd probably have a hard time setting a puddle of diesel on fire with a regular match or lighter, diesel engines use higher compression ratios than regular gasoline engines. Sometimes you can actually even put out a small fire by pouring diesel on it the same way you could with water (I don't recommend trying this) And when you do get it to light, it can be put out with fairly ordinary firefighting foams and such. In an intense wildfire I'm sure there are plenty of concerns about a tank full of diesel in a truck, but significantly less so than an equivalent amount of gasoline.

Lithium batteries, like most EVs use, are kind of heat sensitive, have a lower flash point, burn hotter, and heat can cause some pretty significant efficiency and longevity issues even if they don't catch fire, and when they do they're almost impossible to extinguish.

I'm not saying this to dis EVs, I'm all for them and think this is exciting news, but there's probably some very valid concerns about big battery packs in firefighting equipment that need to be taken into account. That said, the danger of lithium battery fires and other issues having to do with heat and such aren't exactly secrets and I'm sure the people designing these sorts of trucks are well-aware of them and hopefully took those concerns into account, figured out how to minimize the risks, etc. and made guidelines on how, when, and where they should or should not be used (there's a lot of different types of fire apparatus out there, not everything is designed to respond to every type of fire.) I'll admit I haven't read the article, let alone done any research into these particular trucks, so maybe they're even using some other battery chemistry that has less fire risks than common lithium batteries do.