I think he just means disable any of the "advanced" features mentioned above like accessing web content, running custom scripts, etc etc. You might still be vulnerable to any dependencies they use for image rendering itself but I bet it would make it drastically more safe.

Then just have a button for turning those on as needed on a case by case basis

If you disable features then people will see important documents as white rectangles and complain and you'll get a reputation as not working.