That's surface temperature, isn't it?
Rocks in nature aren't heated from below, they are heated by the sun. And in many parts of the corn's natural range, there aren't many rocks to bask on. For instance, my my area of tidewater Virginia, there is no rocky terrain.
UTH's are fine. If that's your choice so be it. It works. My way works, too. No viv is going to perfectly simulate nature. In nature, temperatures fluctuate, sometimes greatly, and much more so than in the typical viv. Snakes in both settings mostly adapt.
But there is no denying that in nature corns don't, as a general rule, burrow to obtain their primary source of heat - except when denning, which captive corns don't do (unless simulated for breeding snakes). They stay on the surface.