[notjoedimaggio]

Perhaps. But "pretty sure you're making this up" is not an appropriate tone or argument to question validity. Especially when the basis for your argument is your singular person experience. Your never having heard the argument in no way counters the original assertion that their instructor had students make that statement, unless you attended every single course and class the original commenter refers to.

Next time, perhaps phrase your argument around your experience. "While I don't know where you're from, I never experienced this. Where I grew up in XYZ, people were quickly corrected regarding their misconceptions."