In general, I agree.
You'd be lucky to kill a few hundred people with one chemical warhead. Physically impossible to kill 300,000 people with one chemical warhead.
Now, I have to disagree, unless we are narrowly defining "warhead". The typical artillery shell or air-to-ground/surface-to-surface missile "warhead" IS too small in capacity to kill 300K people.
There are many types/strengths/dispersal methods of chem weapons. I could easily imagine a 20 gallon tank of some form of persistant nerve agent, sprayed out of a garden sprayer from a helicopter (or from a rooftop on a windy day) across wide swaths of NYC. I'm thinking 300K is a conservative death toll - and 3 million is not out of the question.