His theory is that the large ravine immediately behind the 'sunken road' at Shiloh may have been the catalyst for the term.
Read a book once -- maybe Larry Daniels' -- in which the ravine played a large part in the impenetrability of the "sunken road." The author went into some detail on how some of the troops would climb the slope, hunker down behind the underbrush and blaze away while the balance was down in the ravine cleaning their muskets, refilling their cartridge boxes and taking five. Then some would climb up, others would stumble down and the process continued. The upshot was that every time an offensive formed up, there were relatively fresh troops with clean weapons waiting for them.
And, to horn in further, pretty much all of the famous names we know today weren't in use until the books were written. This included Bloody Pond, Bloody Lane, Bloody Angle, Little Round Top, The Stone Wall, Pickett's Charge, Sunken Road, Hornet's Nest, Peach Orchard, Jones' Field ....
Yes, that's pretty much the way Jeff Gentsch described it as well. In fact, he demonstrated it, by running from the bottom of the ravine to the top, acting as if he was aiming a a gun, "fired" it, then retreated back into the ravine. (His timing was very good - a couple of folks happened to be walking along the road nearby at the time, and when Jeff ran up and went, "POW!," I think it startled them a bit. One of them grabbed his chest and acted like he'd been shot. )
Good point about the names.