The credible first strike capability is not about ability to burn down some enemy's cities by an out of blue attack. The credible first strike capability (AKA Deterence type II, AKA multistability) is the capability to survive his retaliation strike. It depends on capabilities of the first counter-force strike, on capabilities of ABD, on capabilities of the damage alleviation and recuperation.
They believe that they have 80% chance of successful counter-force strike after which the retaliation US strike won't kill more than 5 million of them and 15% chance of less effective counter-force strike after which American retaliation strike won't kill more than 20 millions of them. It's definitely not the chances they are ready take just for lulz, but it is definitely acceptable level of risk when the possible alternative is much worse (like losing Novorossia, Crimea and Sevastopol).