An Office Open XML or ODF file is an OPC package: a ZIP with a content-types map and XML parts. Rename a .docx to .zip and read word/document.xml; in an XLSX the cell values are indices into xl/sharedStrings.xml.
The most common DOCX open failure is a broken relationship part, not a broken document, and relationship parts are visible in the package.
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worked on when the note was written, and those entries are rebuilt with each release of the toolset, so a note can go out of date in a way a format page does not.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.