Also that following bit "no building should ever be more than four stories [...] This seems a completely sensible idea." That's not even near a skyscraper at that point.

Building out means you have less nature, because you have buildings where it'd be. Building out also means traveling that much further to get anywhere. If that includes your workplace, that's a lot of wasted time. A lot of places are also inhospitable. There's also the matter of delays in planning, like for approvals. If it might take years anyway, might as well make it big, along with the other cost savings (per unit).

It's also interesting that they mention "this is Art Deco, the last truly impressive movement in architecture," when you take the skyscraper statement into account, since there's plenty of Art Deco skyscrapers, including the Guardian Building they mention