Do you really think it's important whether this particular poster is 'privileged'? If it came out that she were actually a poor trans orphan who grew up starving and had a severe physical disability, would it change the discussion? What if he were a 22 year old tech bro with a rich dad who chugs beer and red bull and harasses women in his spare time? Are the points he made less correct now? If you think so, you should reconsider how you evaluate arguments, discussions, perhaps words themselves. Does it matter for any real purpose of the discussion other than to help you condescendingly armchair-psychoanalyze him (as you have with me) and tell him that his background is why he believes what he believes, and that he just doesn't really understand and never can?
The beauty of discussions online (and there are plenty of drawbacks as well of course) is that you can express yourself, state an opinion, have a thoughtful discussion with others without people hating or dismissing you for who you are; only what you say. That's why most forums don't have a "real names policy". That's why most forums don't have a 'privilege-meter' next to usernames that uses a sophisticated algorithm to measure how many disadvantages a person has been through, spitting out a final number that tells us how seriously to take her posts. This kind of unsolicited digging into a person's personal life is generally frowned upon in any online discussion forum.
Nuance always matters. Context always matters. You won't derive meaning without them. (And if you aren't trying to derive meaning from something, if you're just aiming for the reptile-brain blood rush of Airing That Opinion--frankly, there are easier methods to replicate that that don't involve bothering other people.)
And while I am emphatically not accusing you of anything in your post, my experience has led me to the rule of thumb that the fear of such a rhetorical dismissal, as you expressed, stems from a desire to dismiss out of hand something challenging. It is in almost every case I can think of a do-unto-others-before-they-do-unto-you defense (even when, as is generally the case, nobody's doing unto you in the first place), so I'd personally think pretty hard before being too holy about it and telling vidarh what's frowned upon here.