It's a plausible story, but so are 1000 other stories.
The problem with the story is lack of facts.
The investment letter isn't named, much less quoted.
The investor isn't named.
The broker isn't named.
All of that smells to high heaven. How can the reporter be sure of these facts and omit to name names?
This is a National Review report which is in turn using the 9/11 Commission report as its source.
If you're arguing that the NR reporter should have gone out and validated every assertion in the Commission report (including the discovery of unnamed sources), that's fine. But so far I've seen many assertions in the 9/11 report where sources are unnamed (presumably for investigatory, intelligence, or even privacy reasons). Should every single unnamed source and assertion in the report be questioned on that basis?
NR is just taking at face value the Commission explanation from the appendix, and fleshing it out with some explanatory reporting. Just as I've done with my own scenario, above.
So we have plausible trading scenarios which are (a) partially explained and (b) make sense. I'd say those stack up pretty well against an insider-trading scenario which is implausible on any number of practical grounds.