I seriously doubt that they can tell reality from the fabrications surrounding an event.
Eh, some can, some can't.
Some are halfway intelligent, and know that their position is untenable. But they like posting on conspiracy boards for the camaraderie and that warm glow from being one of the "in crowd" there - and often they like trolling in more sane forums because it gets them attention.
Some aren't as intelligent and really believe the stuff they are told. "But steel doesn't melt at 1500F so there's NO WAY a fire could have brought the building down!" "Why was the scrap steel secretly shipped to China? It's a conspiracy to hide the truth!"
Remember, we live in a post-factual society, where anyone's opinion is just as valid as everyone else's. "We report, you decide." So if a TV show has a biologist on to talk about how diseases evolve resistance to antibiotics, they give "equal time" to a creationist who thinks that there's no such thing as evolution. And if they have a doctor on to talk about vaccines, then they have an anti-vaxxer to talk about how vaccines cause autism. And in our new "bothsiderism" approach to truth, there's no one side that is right; both positions are equally valid, and it's just one opinion versus another.
Heck, there is now a serious, real Flat Earther movement out there. There are really people who believe that the Earth is flat and NASA, communications satellites, SpaceX, the ISS etc are one big conspiracy. If people believe that, it's easy to see how they would believe in Trutherism.
To paraphrase Trump - "conspiracy theory authors love the poorly educated!"