That 'idea', or similiar has been around for a while.
Clancy had a lot of insights that were interesting. He had a pang of conscience when he wrote the novel The Sum of All Fears and changed details of the construction of an enhanced fission bomb. He was distressed that the 'how to' information was open source and that critical components for such a weapon were available on the open market.
It is true that Fight 77 was headed for the White House...some say flight 93 that crashed in Shanksville, PA...and could not identify it from the air.
True, the idea is not unique to Clancy, however his work was the most readily publicised. i worry about some of the other insights that he and his successor authors, Mark Greaney and Grant Blackwood come up with.