I understand the aim of what's being advocated here, and I really enjoy visiting cities like this (it saves on car rentals). But there's significant reasons people have spent a century or so trying like hell to escape these kinds of cities. Sprawl is a problem, but modeling cities after 15th century Italian towns is a mistake as well. I'd love to live in a place like this I suppose if we're all boutique shop owners and grocers this might work. But most people aren't these days. We actually have to get places that aren't necessarily right in our town.
One of the significant reasons we started moving away from this kind of development, even before cars became common, is that it traps people inside of them and prevents people from getting into and out of them. He uses Barcelona as an example, but he uses the Medieval part of Barcelona, not the 19th century bits.
So now we have to build extensive mass-transit. I'm thrilled as peaches to do that, but it's unbelievably expensive to finance. One of the reasons so many roads get built and expanded is that local governments can finance road development by forcing developers to build the roads or they won't get the zoning for their new mall or suburb (and the increased tax revenue is intended to cover long-term maintenance).
Brand new, above ground, subway/metro runs around $250 million per mile [1] in the United States (built on existing right-of-way). No developer can finance that. A brand new rural/suburban road runs $4-6 million per mile, and $8-10 million in urban areas [2].
I wish that humans had managed to develop a way of life and an advanced economy that made this possible. But we didn't. It sucks, but even if the car had never been invented, cities would no longer look like this.
1 - https://en.wikipedia.org/wiki/Silver_Line_(Washington_Metro)