That might have caught him, you will never know 100%.

Had the U.S. authorities placed him on a "no fly" list he wouldn't have been on that plane in the first place, we know that 100%.

Does the TSA manage to stop every threat coming through U.S. security checkpoints (shoe and underwear bombers originated abroad)? We don't know with 100% certainty.

Have 100% of attempted terrorist attacks mid-air on U.S.-bound flights been thwarted by vigilant passengers? Yes.