all we can do is speculate. If all the report said was something as vague as "that Al Qaeda might try to hijack passenger airplanes", then the report was little more than useless. Every day, there are thousands of airline flights in the US and thousands more overseas. Without any specifics, "Al Qaeda might try to hijack passenger airplanes" could be any day of the year and any country on Earth. You might as well say "there's a 20% chance of rain".

Was the report more specific (assuming it even exists)? Did it say that Al Qaeda might want to deliberately crash the planes into buildings? Was there anything specific, credable, and actionable in the report? Would releasing the report jeopardize any current intel sources that might be useful to prevent future attacks? Without knowing these things, all the rest is little more than idle gossip. Of course, conspiracy wonks seize on nonsense like this, but then, most of them have a very loose grip on reality.