One of the reasons why they are going to have to stay in NYC before moving for some time is to continue to make additional money (ironically). They just don't have enough money yet to retire the way they want and/or not ready to retire. They just don't want live in the South despite my some family members urges to.
From visiting NYC and having family live there I would gander that there are three groups of retirees.
1. The super wealthy that probably have another mansion somewhere else and generally were born into money.
2. The rise the corporate ladder but in debt most of their lives 1980's yuppies (not born into money but have the ambition)
3. The never left the city born and raised and probably have family owned businesses/property.
My parents coming from absolutely zero money are from the second group.
After growing up in "privileged" areas in the ugliest smallest house in the neighborhood with working parents I can observationally say it is amazing how many people that are wealthy that can retire that probably didn't really earn their wealth or at least were significantly aided. There are a lot of trust fund retirees all over and there are a lot of retirees that want to retire like them.
I'm not saying I know for sure the author had/has the silver spoon in his mouth and thinks everyone can afford NYC (group 1)... but simple googling of his name certainly paints a picture of that (his father was a medical doctor and he went to Harvard... maybe scholarship but I doubt it).