Bingo! Another solution using older method in the theme's 'classes' folder:
use \mod_assign\output\grading_app;
class theme_essential_mod_assign_renderer extends mod_assign_renderer {
/**
* Defer to template..
*
* @param grading_app $app - All the data to render the grading app.
*/
public function render_grading_app(grading_app $app) {
$context = $app->export_for_template($this);
error_log('Overridden');
return $this->render_from_template('mod_assign/grading_app', $context);
}
}