You give his "flaw" too much credence.
You think? Maybe that is because it doesn't matter. For, whichever flaw in that simplistic model he may have found - actual or merely invented - it doesn't change the argument. A model can be infinitely tweaked to ensure it more closely aligns with the aspects of reality it seeks to depict. So, no matter the truth value of the asserted flaw, the conclusion - GHE theory is irreparably flawed and thus invalid - doesn't follow.
It all reminds me of Rick Scott issuing a decree in 2015
banning the term "climate change" from Florida officials' language. Another State, also under severe threat from climate-change related changes, I forgot which, ceased collecting climate-change related data altogether. In all cases, the denialists were all giddy, humming "the witch is gone", nothing more to worry about. Of course, down the road they (as we all), and their offspring, will reap the rewards in increased warming and reduced preparedness.
All the while, as you correctly noted, the earth's average temperature should be -18°C, given the radiative flux it receives from the sun. Without the GHE, there's no way to explain the 15°C on average we are actually seeing. Once that is acknowledged, along with the rising CO
2 concentrations, all else falls into place, inescapably.
And yet, all we're seeing and hearing is, "The witch is gone!", nothing more to worry about, and the warmers' underhanded designs on our pocketbooks and control of our lives are thwarted. They even go so far to declare their fealty towards the poor, not usually a group whose interests would rank high up on their priority list, and their inability to make a living in a society heading for a post-carbon future. Cheers!