The title of this story is over the top. The document confirms nothing. It says "someone claimed some flying saucers have been recovered. We decided it's a matter not worth pursuing."
It's a curious document for sure.
The American Southwest is the most heavily atomic-bombed place in the world. There was a lot of radiation in the air back then.
I can just imagine the FBI field agent wiping away the tears of laughter as he types up this memo back to headquarters after chatting with the USAF guys using spatulas to scrape the fried chimpanzees in silver lame' flightsuits off the wreckage of some top secret atomic-powered X-plane.