if you look at the code at
https://www.wp-pizza.com/topic/send-email-on-admin-status-update/

it sets plaintext headers and only what you set the message to be like inside the $message variable

so the message, headers etc you want to send/set you have to create.
if you want to use one of the templates, look at the admin-get-json.php as a starting point
inside

/*************************************************************************************
*
*
*	[order history -> print order]
*
*
*************************************************************************************/
if($_POST['vars']['field']=='print-order' && $_POST['vars']['id']>=0){
/*........code..........*/
}