It's the most obvious counter-factual bald-faced lie imaginable. Absolutely EVERYONE has more than enough information (i.e. there are NO "low-information" people in the audience for this) to IMMEDIATELY respond to such an assertion by regarding anyone able to state such a lie as absolutely INSANE. It's worse than "the sky is green" where one might question the individual's eyesight or language skills.
It's just ASTOUNDING ("Through The Looking Glass" that such an assertion would be regarded as anything but beyond even the furthest fringe claim, being so unarguably false. 9/11 was the single most fatal day of attacks (remember, it was FOUR attacks!) on U.S. soil, BY FAR, in the entire history of this nation! Cheney/Bush CLEARLY had more than enough information to prevent or reduce the fatalities on that day and CLEARLY acted in, at the very least, an incompetent manner by ignoring and suppressing that information.
I regard ANYONE who'd make such a assertion as disconnected from reality to a degree that should qualify them for commitment in a mental institution.
It's beyond bizarre.