Bill Tindall, E.Tn.
If I understand correctly your objection to the method described is that the entry cut might not align with the exit baseline unless the person chiseling was of exceptional skill and experience. Did I not make clear that the chisel is being constrained to the entry baseline, as well as guided, by the top surface of the front jaw of the vise, which in his case was something over an inch deep?
I gave this method a quick trial and the objection you raise is not difficult to overcome. With the broad registration of the chisel on the jaw top surface (simulated in my case by a board clamped across the piece) chiseling a flat dovetail bottom was trivial.
However, there was another issue I did not overcome to my satisfaction. One can not plunge even a 3/8" chisel into the baseline if the waste to be removed is more than a whisker. Rob band sawed out the bulk of this waste as I do. But even taking way more time than I usually do to saw very close to the dovetail baseline I was still left with too much waste to plunge a chisel straight into the baseline with a paring cut (no problem pounding a chisel into the baseline as I usually do).
Rob orally described this technique and obviously some details were lacking. I don't know if he whittled away at this waste until a final paring cut was possible, or he used some sort of shearing cut to make an entry "vee" and then proceed to widen it with the edge of the chisel, or something else. If I encounter Rob again I will ask how exactly he goes about chiseling the waste once the dovetailed board is clamped into position.