The difference is not the toxicity of the fumes, it’s the duration of the fire. A respirator will not fully protect you from a seven hour underground fire.
In an EV fire, you have an initial fire that burns hot and releases lots of fumes (a large part from all the other crap in the car combusting), and then you have a very long period where you have to dump water on the battery to keep the internal oxidation under control by cooling it. The later part is much less dangerous for the fire fighters.
This actually reinforces my point. The first part releases “lots of fumes” into a poorly ventilated underground space, which then requires firefighters to remain for “a very long period.” Your claim that the later part is less dangerous assumes that fumes have dissipated.
