[dang]

"What are you talking about?" is a swipe in online English. It insinuates that the other person doesn't know what they're talking about.

That's even more true in a comment like the GP, which starts with 3 tendentious sentences hammering the other person's point and then moves to "What are you talking about". Such a comment pattern-matches to a cross-examination style of conversation, which the HN guidelines ask you to avoid because it's not what we want here.

If you really want to know what someone is talking about, "what are you talking about?" is an ineffective way to say so, since it means the opposite of what it says, with an overlay of insult.