This, and also the fact that they reinforced and lock the cockpit doors from now on.
The TSA has not stopped ANY attempts at bombing or hijacking airliners since 9/11. Various other methods have, but the TSA has been singularly useless.
Reinforced cockpit doors do sod all. Even without a reinforced cockpit door the crew could have kept them out of the cockpit if they wanted to using a co-pilots foot .
What has made us tons safer after 9-11 is that now there would be reasonable quantity of the passengers who would challenge the hijackers, as recently shown on a French train. Previously most air hijackings were about taking hostages and using them to plead for some worthless chum of yours to be released, as soon as it became clear that the hijackers were never interested in your survival or their own it made trying to subdue them the safest option, no matter how dangerous that seemed.
If you wanted to fly a plane into a building now you would have to steal an empty one first.