Was their argument rigorous or obviously irrational? On the spectrum of plausibility from lizard people to JFK, 9/11 doesn't strike me as especially delusional. I believe we sometimes don't differentiate well enough between abnormally suspicious people that act rationally from the clearly deranged. Non-consensus views could be a sign of either but a failure mode a different set of smart people have is to write off anything non-consensus merely because it's low status.