It's worse: ignorance is humanity's curse and argumentativeness has its good signs, but assumptiveness is trash on both moral and epistemic levels. And the user in question is being assumptive, as he's vomiting certainty over a group of people who he really don't know.

Another character flaw (yup) that the user is showing is lack of basic reading comprehension. As OP hinted it's that kind of user who should be left behind.