That's not how it works. MAD requires an existential threat to a country. Not simply the ability to maybe take out one city. You have to have the capability to completely destroy a country for MAD to take effect.

You also need second strike capability otherwise that one nuke is a target. Russia figures out where Ukraine's nuclear weapons are location, they do a first strike. There's a reason why most nuclear powers have submarines capable of nuclear strikes. Hard to take them out with a first strike.