It's worse than that, it's feigned surprise. "I disagree" is at least intellectually honest, "Huh?" is pretending that the position is so far off that they couldn't even understand it.
I learned the term "feigned surprise" from Recurse Center's social rules[1]. It's related to, but not exactly the same as, that well-known XKCD about "today's 10000", too[2].
> Is this a developing slang or have I just been under a rock and only noticed it now?
It communicates disagreement and pushback to perceived aloofness.
Its archetypal form would be an academic making a Monty Python Witch Trial argument being refuted by an average Joe. You generally don't see it followed by a rigorous intellectual argument, because by its framing it's rejecting intellectualism. Put another way, it's a call to common sense. In reality, it tends to reflect an inchoate argument,.
I can't say for certain about it's commonality online in written form though.