because, according to the Permanent Mission of the Russian Federation, nuclear retaliation is based on multiple sources that confirm an actual attack.

Given that this is coming from the sort of people who thought that setting up the Dead Hand was a good idea, and given that ass-covering and telling the public less than the truth was standard operating procedure in Russia, and given everything we know about the American government's incompetence, paranoia, greed, destructive experiments & actions (like setting PAL locks to zero, to pick a nuclear example) and that nuclear authority really was delegated to individual officers (this and other scandalous aspects came up recently in the New Yorker, actually: http://www.newyorker.com/online/blogs/newsdesk/2014/01/strangelove-for-real.html )...

I see zero reason to place any credence in their claims. This is fruit of the poisonous tree. They have reason to lie. I have no more reason to disbelieve Petrov than other similar incidents (like the Cuban Missile Crisis's submarine incident).