If someone talked to me this way in the real world it would be the last time I would ever interact with them. It’s a Redditism used to express incredulity that the parent commenter could be so stupid as to have posted what they posted. It adds nothing to the conversation and doesn’t belong here.
You're reading a lot into two words there. And someone probably has said "um, what?" to you in the real world; but you wouldn't notice because it is a common pattern of speech.
The internet will be a remarkably combative place if that is the standard you set for when someone calls you stupid - it'd be a lot better to only read that into a statement if it actually gets explicitly said.
No, it's not, it's among the same vein as "huh?" - it's a thinly-veiled attempt to paint opposing view points as so stupid that you literally cannot comprehend them.
But you can comprehend them, we all can. So it's just abrasive and annoying. You can express a conversational tone without being an asshole. You don't need to act like your conversational opponents are super far out there or crazy or whatever.
