He realised it was a system glitch, because it only detected a singular US nuclear missile.
He realised the US would never ever (intentionally) launch a nuclear attack on russia that was just a single missile. MAD meant they would either launch an all out devastating attack to eradicate everything, or launch no nukes at all. Anything in between those two made no tactical sense whatsoever, and, at least back then, the US millitary wasn’t run by complete morons.
(Also, in this case the risk of not responding is relatively low. A single missile, even if real, is relatively easy to intercept, and even when aimed at the capital, is not got going to take out the entire command structure and launch capabilities, so even if it turns out the attack was real, Russia would still be able to retaliate after it’s conclusion, which would not be the case with an all out attack)
Yeah, it’s totally true, everyone knows the only member of the allied forces was actually the USSR. No other country fought against the Nazis, they did completely single handedly, and totally wouldn’t have just sat there and let it happen, if Hitler hadn’t violated their non aggression pact first.