The subtext of the comment you're replying to is that the cities people want to live in are all full up, and people who want to live there should just go make new cities somewhere else.
Zoning keeps every city from being denser than it could be. Zoning is the problem. There’s no such thing as a city that is full up.
They really aren’t though. Even New York is filled with tons of somewhat wasted space and excessive car infrastructure. Especially in the outer burrows.