It means opening PNG files, saving PNG files, creating PNG files, etc without seeing XCF at any point (except perhaps if you try to use a feature - like layers - not supported by PNG when saving and GIMP showing a dialog to warn you about it).

Replace PNG with any other file format (outside of XCF, of course).

The technical side is irrelevant, i am talking about the UI and its behavior here, not how it could be implemented.