An INDD file has a 16-byte signature and a private, version-locked structure: a layout saved by InDesign 2024 cannot be opened by InDesign CS6. IDML was designed to solve exactly that.
IDML is an OPC package of XML parts, so it can be inspected in any ZIP tool. It does not carry the links, which is why a layout arrives with missing graphics unless the links were packaged with it.
Where this came from
Notes like this one come out of the file library rather than out of a release schedule. The pages linked above are the entries that were being worked on when the note was written, and those entries are rebuilt with each release of the toolset, so a note can go out of date in a way a format page does not.
When that happens the note stays where it is and gets a correction at the bottom rather than being rewritten, because the point of most of these notes is the order things were tried in. The editors who write them are the ones who maintain the catalogue.