>the only way for that energy to be expended is in either thermal energy
Correct, much of the atmosphere would become superheated, perhaps to plasma, and radiate the energy away non-directionally. So only half would radiate toward the surface -- not that that's a whole lot of help.
Note that for an asteroid reaching terminal velocity, that's already largely the radiative pattern for thermal energy and the bulk of the energy transfer. That is, for Chelyabinsk, roughly half the heat radiated directly to space.
For a sufficiently large single impactor ... there's also an ejecta cloud, and for sufficient sizes (e.g., Chixulub+), that also results in atmospheric warming by a similar mechanism, though from secondary rather than primary fireballs. Several descriptions of the K-T mass extinction go into this mechanism in detail. Even at the antipodes, most exposed terrestrial life would have been subjected to at the very least uncomfortable, and probably lethal, heat.
I've had the experience of feeling radiated heat from a bushfire (and a relatively small one at that), and even at 100s of meters distance, it could be distinctly felt, with the sense that this wasn't just a nearby small heat source (a radiator or firepit), but a large conflagration. The prospect of a wall of flame sweeping down a hillside, or of a sky on fire, is slightly more real to me since that.
Correct, much of the atmosphere would become superheated, perhaps to plasma, and radiate the energy away non-directionally. So only half would radiate toward the surface -- not that that's a whole lot of help.