Apart from the specifics about 9/11 (which I don't think was a conspiracy, but I don't know enough to claim to really know), I agree wholeheartedly with this post. It's a common, or at least plausible, tactic: blame a big group of people, then claim that "specific subgroup B didn't do it, therefore nobody in the group did anything".

For added points, blame a group of people that barely intersects with the actually responsible group, then get the discussion tied up over which subset of your blamed group was actually responsible.