Multiple governments in multiple countries over multiple decades, all with effective programs to prevent the exposure of evidence which could be used to their advantage, or at minimum could be considered as to their advantage.
It does not pass the smell test. The Soviet Union despised religion, and could have used this to attack religion. They never did.
“The Soviet Union despised religion, and could have used this to attack religion.”
And thereby givng up secrecy. And what does religon have to do with this subject?
You must not be familiar with Genesis 6.