I feel like this page does a good job of telling me nothing. It's a representation of a graph? Where each node can be a text string or a file or a URL or a container of other nodes? And somehow these links and files and containers have a color? What is the purpose of this? Why would I want my file to be blue?
It's a reinvention of SVG with all the same problems. JSON is not any better in terms of data efficiency or fast updating. It's like these people have never done graphics programming.
Perhaps the home page is more useful than the spec to explain the purpose of the file format: https://jsoncanvas.org/
See also the original inspiration for the format: https://obsidian.md/canvas