Earlier quoted context omitted.
Nuclear capability wouldn't necessarily rule out this kind of a decapitation strike, it's just that it's very hard to imagine this kind of an operation actually being successful in any nuclear-capable country. The US couldn't just fly a bunch of helicopters to Pyongyang or Tehran and do the same within 30 minutes. Most likely every single one of those helicopters would end up being shot down.
Nuclear capability by itself isn't a complete deterrent. It has been widely reported that the US military has made contingency plans for a decapitation strike and seizure or destruction of nuclear weapons in Pakistan in case the situation turns really bad there. Real deterrence requires a credible second-strike capability on survivable platforms such as submarines.
Similarly, if a head of state is killed by poison or other similar means, you could hardly expect nuclear retaliation when their successor later discovers what happened.