Welcome to cruise-chat.
The people you mention managed to sneak by without getting caught. Security and pool deck attendants do monitor it. These are not cruise line rules. The prohibition on non-potty trained children comes from the Center for Disease Control's (CDC) Vessel Sanitation Program (VSP), which all the cruise lines have to comply with. Swim diapers are not leak proof. I've been on cruises where pools have had to be drained because of violations, and the crew spent many hours manually scrubbing the walls. Not fun for them or the passengers.
On the Freedom and Libery Of The Seas they have a baby splash zone which does allow non-potty trained kids but that is it.