Debt (or more generally, an economy based on promises) is what carries essentially all of modern civilization.
Going back to silver and gold coins takes us back to Ancient Greece. It works, but it is very inefficient and very little gets done by comparison.
And even so, metal coins aren't actually the goods they're traded for, so you're still relying on a promise of sorts.
A gift economy would be the truly promise-free way of life. There's a reason we have very little knowledge of how those work: they don't produce much worth remembering for future generations.
Unfortunately the author doesn't talk about any of these points.