If they need to move a million people from a location, they can just make it happen. This isn't San Francisco we're talking about, Chinese authorities can snap their fingers and dictate policy. They can point at an area and that's where the new expansion will go for the city. Zoning problems? Nope, it's all flexible depending on who wants something done.

There's no need to build so far in advance if you're China. They're front-loading economic activity, attempting to pull future returns forward. You can tell because the returns they generate on their debt has plunged drastically over the last decade. If they were still generating the old tremendous growth returns on their borrowing, it'd be a different story, then they'd be feeding growth. Instead, they're stealing from the future (which will have to pay for all of this low value forced expansion).