The person presenting the the rules did so in a rational fashion. The TSA employee probably in the back of his mind remembered "Don't photograph the screens". This happened once. Is it likely behavior for a terrorist to sneak through or call extra attention to themselves?

There have been many Joe and Jane travelers who have showed up at the CP with printouts of the web site, which documented legitimate rules, and were told to go and get stuffed. No numbers exist on how many times the events in the video have occurred at a CP where Joe or Jane brought the rules, and the TSA employee followed the rules after bring reminded.

If someone probing the CP does this, it's no guarantee that they will get the same result from TSA. If they actually try and create a piece of paper that says they can bring in something explosive there's still a chance that ETD will catch it, assuming the bag X-Ray doesn't show something weird. If it's a non-explosive prohibited item they intend to use, the cockpit door and passengers/cabin crew will make this likely to fail.

If we call common sense a security issue, we can kiss common sense good bye.