I mean, it’s true that solid (not powdered) magnesium is safe to use because it’s not going to spontaneously burst into flames on its own from high temperatures alone, but that’s not what we’re talking about. We’re talking about what happens to it after the car is already fully engulfed in flames from another source. I’ve been doing some reading about it in the last 10 minutes, and the sources I’ve found have pretty much been in agreement that when exposed to flame, the temperature at which the magnesium would combust is lower than the one at which it would melt.

For example, from https://firefighterinsider.com/magnesium-flammable/ :

Magnesium… will ignite at temperatures of around 630 degrees Celsius or 1166 degrees Fahrenheit.

…

Magnesium melts at about 1,202 degrees Fahrenheit or 650 degrees Celsius.

In most cases, it will ignite before it melts and molten magnesium is something that would need to be made under strict conditions in a lab.

My guess is that the re-solidified pool of metal is probably close to 100% aluminum or maybe a mix of aluminum and steel, but wouldn’t contain much magnesium. (Or rather, the magnesium it would contain would be particles of MgO (magnesium ash) suspended in it, not metallic Mg.)

I suppose if we’re talking about aluminum-magnesium alloy car parts instead of pure magnesium ones, the eutectic nature of the alloy might cause the pieces to melt before they caught on fire. But once it is melted, I’m not sure being mixed with aluminum would continue to stop the magnesium from igniting.

See also:

• https://www.hooniverse.com/magnesium-in-automobiles-smart-car-sparkles/

• https://profoundqa.com/does-magnesium-melt-or-burn/

• https://chemistry.stackexchange.com/questions/74367/how-do-you-melt-magnesium-without-causing-its-combustion