Flow and the dumps store user info as user ID or user IP address.

With the current patch, if you import a revision for a user that doesn't exist, it basically kind of just drops the user string, but everything works.

However, it should probably import by username and use user_ip if there is no user by that name. This is what core apparently does ( https://github.com/wikimedia/mediawiki/blob/master/includes/import/WikiRevision.php#L463 ) , except they call it rev_user_text, whereas we call it rev_user_ip which makes it look worse.