rebuildRootPackage() still references drupal/core 8.0

The rebuildRootPackage() in src/PackageManager.php still references 8.0 as the version of drupal/core to be used, even though the comment says it should use the version from Drupal core's own composer.json file, which is 8.1 in my case.

    $root_package = JsonFile::read($this->root . '/composer.json');
    // Initialize known start values. These should match what's already in
    // the root composer.json shipped with Drupal.
    $root_package['replace'] = [
      'drupal/core' => '~8.0',
    ]

The code seems to do the opposite from what the comment just above it states... Is there a reason why it overrides the value provided by core?