Disable SKU attribute in admin Magento 2

Question

I'm trying to make product SKU attribute not editable in Admin Products->Inventory->Catalog.

I'm using this previous response as an example: Magento read-only and hidden product attributes

I've created an Observer to use lockAttribute("attribute_code") method.

Here is the Vendor/Module/etc/adminhtml/events.xml:

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="catalog_product_edit_action">
        <observer name="vendor_admin_lock" instance="Vendor\Module\Observer\ProductLockAttributes" />
    </event>
</config>

And in ProductLockAttributes.php:

class ProductLockAttributes implements ObserverInterface
{
    /**
     * @param \Magento\Framework\Event\Observer $observer
     * @return void
     */
    public function execute(\Magento\Framework\Event\Observer $observer)
    {
        $event = $observer->getEvent();
        $product = $event->getProduct();
        $product->lockAttribute('sku');
        $product->lockAttribute('sku_type');
    }
}

The event gets called but in the admin interface when editing a product the SKU is still editable.

Can anyone tell me what is wrong with my code?

Thanks.