For whatever reason this keeps getting thrown around as though it's true, possibly because it's a bit scary to see a radically different system that is by some measurements surpassing our own.

It's true some are empty, at least for now, but many aren't.

For example "Yangtze River Delta Megalopolis" was approved in 2016 and now has 240 million people.

https://en.wikipedia.org/wiki/Megalopolises_in_China