It is pretty simple that you save the render content of KoolReport to a variable and then echo in the view of smarty:

$report = new MyReport;
$report_content = $report->run()->render();

You assign the $report_content to the template data of smarty and inside view of smarty you do:

<div>
{$report_content}
</div>

Ah yes, I forgot about that. You are right it should be ->render(true) for content to be returned. It should not render there in controller if true is passed to render() function. Not sure if there is any special settings that causes this behavior. I will consult dev.team and come back to you.