No, the Doc is buggy. In stable5 the username was returned, too, see https://github.com/owncloud/core/blob/stable5/lib/user.php#L470 The internal login method depends on it, too: https://github.com/owncloud/core/blob/stable5/lib/user.php#L244

This is also what is returned before this PR: https://github.com/owncloud/core/blob/master/lib/user.php#L422