The point is more about even countries with a minimal nuclear arsenal being in relatively equal military footing as the countries with large arsenals because no country seems willing to risk losing one of their major cities to nuclear attack. So, threatening 100 cities is about the same as threatening 1 - no one will attack you.
That's pretty much the main reason north Korea hasn't been attacked since the Korean War: even without nuclear weapons, nobody wants to risk war when Seoul is all but guaranteed to be leveled.
Apparently there are backup systems in both countries that will fire nuclear weapons automatically (!) at predefined targets if certain cities are hit with nuclear attacks - e.g. Washington, Moscow, etc.
That's why deterrence is effective - because any strike can automatically lead to hundreds of millions of people dying (and, if the modelling of nuclear winter is correct, potentially pretty much all large mammals within a few years)...