Either the docs are incomplete, or the auto-installer is. It looks like pkp-lib has Composer dependencies which prevent the auto-installer from running correctly:
PHP Warning: require_once(omp_dev/lib/pkp/lib/vendor/ircmaxell/password-compat/lib/password.php): failed to open stream: No such file or directory in omp_dev/lib/pkp/classes/security/Validation.inc.php on line 16, referer:
I’m happy to file an issue or to offer a fix, but I’m not sure whether it’s preferable to ask the administrator to run Composer, or for the installer to Just Do It. It looks like the previous distributed releases either didn’t use Composer, or wiped out its traces after gathering the dependencies…