Simple explanation: Most people are uninformed and irrational.
Saying someone is uninformed or irrational isn't the same as saying they're mentally ill or unintelligent. Your comment, however, does well to lend credence to their anecdote.
https://www.merriam-webster.com/dictionary/irrational
(1): lacking usual or normal mental clarity or coherence (2): not endowed with reason or understanding
Perhaps the next time you feel the need to jump into a conversation to teach someone a lesson, take a moment to consider that you might be the uninformed one, because it's a distinct possibility based on this anecdote.