G Webb is not guessing. He is receiving information from sources that know.
Diplomatic pouches started out just as their name refers, pouches belonging to diplomats. Small pouches belonging to diplomats were off-limits to inspections.
Since their initial status, the pouches have become increasingly larger to encompass luggage and trunks, and today entire sea containers, and not just one container but many containers per ship.
On any one particular day, there are thousands of container ships at sea. With many such ships containing many such ‘diplomatic pouches’ that are exempt from inspections and inquiries, it is seen that illicit, illegal traffic is taking place on a vast global scale.
In short, G Webb is exposing an enormous loophole allowing for secret shipments.