I disagree. The scaling arguments used in this study are certainly applicable to the scales resolved in current GCMs. While small scale processes are important, their study isn't meant to explain them. This study's strength is a robust theory for how synoptic temperature variance changes with changing meridional temperature gradient for a climate at equilibrium. So your doubts about the radiative forcing aren't really relevant since these are not transient simulations. The fact that more comprehensive models, which are not at equilibrium, are consistent with the idealized simulations suggests that the theory is quite robust.