This was a fun rabbit hole to go down. Magnesium does not seem to be present in radioactive fallout, there is more Manganese than magnesium from the data I was able to find online. Further NOX, the chemical combination of oxygen and nitrogen is a serious pollutant by-product of nuclear explosions indicating nitrogen seems to remain largely elementally unmolested by the blast. Finally, looking at the calculations it seems to fuse nitrogen nuclei in any kind of meaningful number would require atomic detonations with heat in excess of tens of billions of degree, while real contemporary explosions produce only hundreds of millions of degrees.

The other claim I wondered about was that a nuke created the hottest temps ever on earth. Wouldn't a large asteroid strike come close? Edit: chat g 4 set me straight, nukes are tens of millions F, asteroid impact thousands of degrees F.