These people were gloriously confidently incorrect. There is a fine line between believing in yourself, and being so cocky that you don't stop and question any single thing that goes on in your head at all. After all, they say to be wise is to know what you don't know, and by that logic these people are some of the dumbest around. (Which they are, obviously, because of how wrong they are.)
Perhaps even the greater folly than believing without question that your feelings must be correct, is the burning desire to correct everyone else in their feelings that of course, since they are the opposite of yours, must be wrong. At no point do they stop and think that something might just be wrong with theirs. The crushing of this overbearing hubris is what brings us so much joy in watching these people from r/confidentlyincorrect get utterly and completely schooled.