How well does it handle earthquakes?

It doesn't.

Earthquake resistance is increased by strengthening the concrete, but it mainly depends on how the structure is built, not just the shape but additoinal measures used to strengthen the structure. We've been stuck on rebar for so long that research on alternatives has stalled, but there are interesting opportunities and developments in that area. For example, basalt fiber and bamboo have potential.

Sustainable, innovative building structures that don't require energy-intensive and corrosive steel will depend on a convergence of strengthening technologies and archtectural techniques, maybe even including ones like described in the original article.

Considering all the environmental and social problems we have with modern housing and construction, this is a prime area for innovation!

https://www.architectmagazine.com/technology/two-natural-reb...

Isn't there a massive difference in wall thickness between what the Romans did and what we do today?

Modern uses of Roman concrete go beyond what the Romans did. We can create completely different types of structures using the same principles. Here's an example of recent developments in research:

https://www.sciencedaily.com/releases/2021/01/210113090920.h...

Concrete without rebar lacks tensile strength, and Roman concrete is no different. When using materials with insufficient tensile strength, you're severely restricted in the kinds of structures you can build. That's why Roman buildings used so many domes and arches, and why they were massive.

Practical Engineering has a good video about it: https://www.youtube.com/watch?v=qL0BB2PRY7k