Drupal\Core\Field\ConfigFieldItemList tries to use an inexistent FieldInstanceInterface

Drupal\Core\Field\ConfigFieldItemList has this use statement:

use Drupal\Core\Field\Plugin\DataType\FieldInstanceInterface;

and then does a check on it in the constructor, which will never return true:

  public function __construct($definition, $name = NULL, TypedDataInterface $parent = NULL) {
    parent::__construct($definition, $name, $parent);
    // Definition can be the field config or field instance.
    if ($definition instanceof FieldInstanceInterface) {
      $this->instance = $definition;
    }
  }