XML would require a schema to express the concepts in Preserves, or JSON for that matter.

The reason JSON is lower friction than XML for data representation is that you get basic data representations (numbers, strings, arrays, maps) for free in a natural native syntax that happens to parallel multiple programming languages.

XML, in contrast, is a meta-language that allows schema to express different data representations. You've got to use attributes and elements to represent data and data types. XSD is a common datatype schema, but it's quite verbose, and data serialization looks very different from what it looks like in a programming language representation.

Preserves looks like a superset of JSON. It includes additional data representation concepts through syntax extensions, but the idea is the same.

What I don't see is a standard way to map record types (like "irl" in the tutorial) to a unique identifier like an URI/IRI, or something like a CURIE. That kind of feature would allow Preserves to better describe standardized record types.

XML would make a fine choice. It lacks atomic data types other than text, and compound data types other than sequences, unless you count element attributes, which are in a kind of awkward position because of the historical development of the language. Preserves has a richer suite of primitive data types and decomposes XML's elements into separate notions of map, sequence, and tagged value.