Is anyone arguing "all fringe ideas must be false" or, conversely, "widely accepted ideas are true?" That's basically just argumentum ad populum, and I don't think that's the argument of the paper. Rather, it's that those with fringe ideas tend to be over-confident in their beliefs.
IME, a scientific/empirical mindset tends to lead one to a state of epistemological modesty, in which few things are unassailably true, and beliefs are provisional until disproven. One crucial feature of that mindset is the notion of falsifiability. Knowing if/how ideas may be falsified helps one avoid leaping down conspiracy rabbit holes. <- (A likely unfalsifiable statement!)
IME, a scientific/empirical mindset tends to lead one to a state of epistemological modesty, in which few things are unassailably true, and beliefs are provisional until disproven. One crucial feature of that mindset is the notion of falsifiability. Knowing if/how ideas may be falsified helps one avoid leaping down conspiracy rabbit holes. <- (A likely unfalsifiable statement!)