Their objection is demonstrably incorrect, as Bruenig has already pointed out. Having established that, a second thing worth pointing out about their objection: it's hilariously obvious. It's so obvious that David Henderson didn't even feel the need to articulate it directly, and instead spent his "Spot the Problem" blog post dropping smug hints. It's so obvious that Scott Sumner explicitly asks, "How could Bruenig overlook the obvious?"
Sumner seems to think he is being rhetorical here, but if he had taken his own question seriously he might have noticed a problem. Insofar as his point was obvious, Bruenig almost certainly did not overlook it. If Henderson can merely gesture towards it to a general audience and expect them to know what he's talking about, it's relatively unlikely that a prolific scholar on the topic would be in the dark. Snark about how progressive brains might be wired differently may make for self-indulgent ridicule, but it's hardly a credible defense for this theory that Bruenig missed his point.
I bring up the incident because it reminds me of a similar episode that took place last week:
"How can you have the wobbling of the earth cooling the earth, but it not be included in any projections [of climate change]?" - Rep. Steve Stockman [R-TX]
The answer, of course, is that wobbling has been accounted for in the projections. As they have with cyclical warming, volcanic eruptions, and all of the other obvious and easy to understand factors that climate science deniers like to pretend the scientists haven't noticed. Stockman is relying on the same ploy that Henderson and Sumner rely on: appealing to an objection in all of its obvious self-evidence, while simultaneously suggesting that his critics have somehow missed it.
All of this plays into the psychology of the counter-Enlightenment - which foments contempt for scholarship (to the point of ridiculing reading) while fetishizing the semi-erudition of unearned knowledge. The right cultivates intellectual arrogance, insisting that people who have dedicated much of their lives to particular fields of knowledge are just "ivory tower elites" who don't actually know any more than the rest of us.