I love the idea of Solid and firmly believe that a user should always own and be able to access and manipulate their data in any application they are using.
However beyond syntax, semantic interoperability is always going to be hard - maybe thats why we have standards organizations! I remember the days of CORBA, and XML/SOAP/WSDL (then REST, JSON etc), there is no silver bullet. Except in specific narrow domains its hard to see how there can ever be serendipitous interoperability.
Within specific domains tho maybe theres hope. I've always been a plain text fan and am pleased to see the common usage of markdown in a lot of modern note-taking apps - there's clearly interoperability possible between Obsidian and LogSeq with common markdown files.
The TODO app example from the article is an interesting space. There are thousands of todo/scheduling/productivity type apps. They should be able to plug and play from a common data store. I'm personally banking on using org-mode as the data store for my BrainTool app. Its thus interoperable with the emacs org ecosystem and other tools like LogSeq.
Org-mode is human readable and covers all the basics of headings, todo's, links, timestamps, tables, checkboxes etc. I'd love to see an ecosystem of serendipitously interoperable org-based productivity and PMK products!
For those unfamiliar with the author, they're the developer of the most prolific apps created to work with Tim Berners-Lee's Solid: https://solidproject.org
Why not xml and xml schemas?
In fact you can use XML, but not barebones XML. XML does not contain semantics, which is needed to achieve interoperability. So you should use some more specialized XML, namely, RDF/XML. Which of course will increase complexity to XML. You can use also JSON-LD, which is a more complex JSON bc it adds semantics.