Some days ago, while helping in the Italian Community I've realized that https://docs.moodle.org/33/en/Backup_and_restore_FAQ#How_can_I_extract_original_files_from_a_Moodle_backup_file.3F could still work but it is not technically correct, strictly speaking: the mbz file is a tar.gz compressed archive and not a zip archive (since 2.9, -MDL-49298, and experimental since 2.6, --MDL-41838-).

Is it a missing or a choice, e.g. to discourage the hacking of a Moodle backup file?

See the details provided yesterday in https://moodle.org/mod/forum/discuss.php?d=352706#p1426016: it could be written better e.g. to use the z option, there it was done to help the reader to understand the "internals" of a tar.gz archive.