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UPDATE:
Latest setup outputs ID's but not comma separated. They appear like 1-item individual arrays 😕 like "ID1""ID2" instead of, "ID1,ID2".

Also, HTML markup updated. Used same syntax for the "contents:..." bit, and here it outputs correctly...

PHP:
 

public function initContent()
	{
		parent::initContent();
		$order = new Order((int)($this->id_order)); $products = $order->getProducts();
		$product_ids = array();
		foreach ($products as $product) 
  		$product_ids[] = (int)$product['id_product'];										
			$this->context->smarty->assign(array(
			'$products' => $products,
			'id_order' => $this->id_order,
			'is_guest' => $this->context->customer->is_guest,
			'HOOK_ORDER_CONFIRMATION' => $this->displayOrderConfirmation(),
			'HOOK_PAYMENT_RETURN' => $this->displayPaymentReturn()
		));
			$array = explode(', ',$product_ids);
			
		if ($this->context->customer->is_guest)
		{
			$this->context->smarty->assign(array(
				'id_order' => $this->id_order,
				'$products' => $products,
				'reference_order' => $this->reference,
				'id_order_formatted' => sprintf('#%06d', $this->id_order),
				'email' => $this->context->customer->email
			));
			/* If guest we clear the cookie for security reason */
			$this->context->customer->mylogout();
		}
...

HTML:

{literal}
			<script>
		       
		       fbq('track', 'Purchase', {
		       	content_ids: {/literal}{foreach from=$products item="product"}{literal}
						       	{/literal}'{$product_ids}'{literal}
						       	{/literal}{/foreach}{literal}, 
		       	content_type: 'product', 
		       	value: {/literal}'{$_value}'{literal}, 
		       	currency: 'RON',
		       	contents: [
			        {/literal}{foreach from=$products item="product"}{literal}
			        {
			            id: {/literal}'{$product.product_id}'{literal},
			            quantity: {/literal}'{$product.product_quantity}'{literal},
			            item_price: {/literal}'{$product.total_wt}'{literal}
			        }
			        {/literal}{/foreach}{literal}
			    ],
		       });
			</script>