Well, what is really wasted here?
Workers were paid for their work, suppliers were paid for their supplies; that's all good stuff for an economy. Was labor pulled away from better projects? Doubtful. Is there a shortage of building materials in China that would prevent a better project from being built? Also doubtful.
Would it have been somehow less wasteful, or more beneficial, to simply let idle labor stay idle (and unpaid)?
I don't really care if this government stimulus is labeled Keynesianism or not, but the original author obviously used the term as a pejorative. Keynesianism has been a tried-and-true tool for governments to goose their economies, often creating ultimately useless stuff (arms during wartime, etc.), but always employing labor and keeping dollars flowing through the economy. It doesn't matter where all of those Sherman tanks are now, and it didn't matter (to the economy) if they were destroyed in battle, sunk en route, or survived to ride in victory parades; what mattered to the economy was that people were employed and commerce was happening.
Anyway... I see no big problem with building ghost cities. It's no more wasteful than any other useless product, and we happily count useless products and services as part of GDP. Your spray tan counts. So does a $100 bottle of wine, a pedicure, undercoating on a new car, etc. Money moves, the economy is happy.