It's not that either. If the journal couldn't make sure it had the right to publish, it wouldn't publish. Whether or not one of the coauthors is formally listed

It's not that either. If the journal couldn't make sure it had the right to publish, it wouldn't publish. Whether or not one of the coauthors is formally listed as coauthor doesn't affect the disposition of the copyright. It's a question of compliance with the journal's formal publication process.