The number naturally has to be adjusted when trying to define 'ghost town' in America or anywhere else because relatively few places are built to be as dense as 10k/km^2.

You can find many sources complaining about this in the US, yes. Real estate speculation, etc. People aren't happy about it, so that description is not unreasonable. Some cities are trying to pass laws to address it (not that those laws seem to work).

The density of construction matters a lot here too. Many US cities like the ones you mention were not planned out and are subject to sprawl.