This shows that anyone with the slightest motivation to do harm would have zero difficulty replaying 911. The reason there aren't more terrorist attacks isn't because various security agencies around the world protect us from them. It's because there are extremely few terrorists.
It’s also just one of those hard things to prove: is TSA actually stopping attacks like 9/11? The simple presence of them might be enough of a deterrent or we might just be extremely lucky. Seems these days the real threat is drunk passengers attacking flight attendants.
So are they stopping anything serious? It's a safe bet they're not.