pre_authorize hook is documented wrong.

I misunderstood this as well.

Actually account is taken from

  public function userLoadBySub($sub, $client_name) {
    $result = $this->connection->select('openid_connect_authmap', 'a')
      ->fields('a', ['uid'])
      ->condition('client_name', $client_name, '=')
      ->condition('sub', $sub, '=')
      ->execute();
    foreach ($result as $record) {
      /** @var \Drupal\user\Entity\User $account */
      $account = $this->userStorage->load($record->uid);
      if (is_object($account)) {
        return $account;
      }
    }
    return FALSE;
  }

So if there is no entry for the user in openid_connect_authmap, $account will be empty event it the user has a actif drupal account.