Lithium metal reacts violently with water to produce LiOH and H2. Lithium ions embedded in something else (a polymer electrolyte for LiPo cells) produce the same products, but at a significantly reduced rate so the reaction can be cooled below its critical temperature. We don't use lithium metal in EV cells (they also tend not to be rechargeable).

https://en.wikipedia.org/wiki/Lithium_metal_battery

https://en.wikipedia.org/wiki/Lithium-ion_battery

I guess cooling is more important than keeping away oxidizers in this case and it's hard to beat water in that regard.

Yeah it reacts with water, but if it is damaged it also reacts with itself and the air around it anyways. Adding water still cools it which slows the reaction rate and reduces overall fire temperature. Once a lithium battery lights on fire there really isn't much you can do to actually stop it, you just want to suppress it and keep it from damaging or lighting other things nearby on fire.

I don't believe lithium-ion cells have lithium metal in them.