when someone uses a personal experience to bolster an argument, it is appropriate for someone to question the validity of how likely it is. It's not an ad hominem or a personal attack.
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."
Actually his/her comment was anecdotal, in that no one can verify. My comment is a noting a shared cultural understanding that everyone confirm or reject. (specifically: the likelihood of people saying your classmates are genetically inferior is taboo) It's not anecdotal.
"I think those claims are unsubstantiated due to lack of sufficient evidence" or even "I think those claims are unsubstantiated" alone comes across better than "pretty sure you're making that up".
Best not to be 'pretty sure' with accusations, even mild ones, especially when you're apparently wrong. And why are you pretty sure? Seems like you have some reasoning, mention why with your accusation or your viewpoint will be 0 on the contribution scale. It comes across like a child discussing whether or not he thinks Santa is real "Yea I'm pretty sure he exists and you're making it up"
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."