5. After the 1999-2000 millennium alert, ... Clarke held a meeting of his Counterterrorism Security Group devoted largely to the possibility of a possible airplane hijacking by al Qaeda... [T]he possibility was imaginable, and imagined” (345).
Page 265
The 9/11 Commission Report: Omissions and Distortions
David Ray Griffin

This is a little vague. What type of aircraft? Hijacked where? To do what? Here’s the context of this as it appears in the original text:

In late 1999, a great deal of discussion took place in the media about the crash off the coast of Massachusetts of EgyptAir Flight 990, a Boeing 767. The most plausible explanation that emerged was that one of the pilots had gone berserk, seized the controls, and flown the aircraft into the sea. After the 1999 -- 2000 millennium alerts, when the nation had relaxed, Clarke held a meeting of his Counterterrorism Security Group devoted largely to the possibility of a possible airplane hijacking by al Qaeda.17

In his testimony,Clarke commented that he thought that warning about the possibility of a suicide hijacking would have been just one more speculative theory among many...
Page 345
9/11 Commission Report

The context here suggests a possible suicide hijacking, however we don’t know of what type of plan, or where, or what form the attack might take. In addition, we have the issues itemised above: we don’t know how many other issues Clarke considered, for instance, and we’ve no reason to believe his ideas were considered any more likely now, than earlier. It’s hard to see how this can come close to contradicting the NORAD position.