It will be 50 sq. km by the time the extension is finished [1].
[1] https://twitter.com/TaylorOgan/status/1859149340897628545
The wording is confusing, but I believe the Zhengzhou International Land Port is 50sqkm, not the factory. That also fits the numbers they're giving [1]: "Zhengzhou International Land Port Project announced ——total area of around 50 square kilometers", and that includes a lot of "open-air warehouse" (parking lot). It's still massive, but not quite as crazy.
Agreed, the factories clearly aren't the same size as Manhattan.
But as a comparison of scale, both the airports in Denver and Dallas-Fort Worth are larger than Manhattan (27 mi2 or 70 km2).
https://en.wikipedia.org/wiki/Dallas_Fort_Worth_Internationa....
While that is a ton of activity (and empty space, if you have ever seen those airports), the Big Apple might not be the best reference for scale.