Thought I would update this thread with the solution.
It turns out that this particular year / model of Sorento does *not* have that star-shaped coupler in the steering column. My mechanic found 1/8" of play at the end of the intermediate shaft where the splines interface with the rack-and-pinion assembly. The pinch bolt at that end was tight, and I didn't see any obvious wear on the splines on the part he removed. The attached picture illustrates.
He replaced that shaft, and the rattle is gone. MUCH better.
Cost for the genuine Kia part was $250, and $150 labor to install.
Finally, an interesting note the mechanic mentioned. He's worked on a number of the Kia coupler "rattles," and he said the symptoms / sound on my Kia were basically identical to those.