Conspiracy theories like 9/11 were early instances of the communication distortion that brought us Donald Trump. The web lets anyone say anything, and there's no experts to check it. Some people of good will thought they heard or saw strange things, and those with a motivation to see a conspiracy have pounced on those.
As for whether we missed warnings and opportunities, that's a very different question. As to whether it was appropriate to whisk out of the country the Saudis is another question, yeah misuse of power. Yeah, there's lots of power being wielded for the ends of the rich and not the welfare of people, but that didn't extend to blowing up buildings.