A tar header is 512 bytes and the magic ustar sits at offset 257, which means the first bytes of a tar archive carry a file name, not a signature. Tools that look for a signature at byte 0 report the file as unknown.
GNU tar uses a long-name extension that BSD tar does not, which is why a .tar written on Linux sometimes refuses to open in a Windows tool. Re-pack it as .tar.gz or as a ZIP and it opens everywhere.
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worked on when the note was written, and those entries are rebuilt with each release of the toolset, so a note can go out of date in a way a format page does not.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.