According to the one rep max calculators I have found, and using round numbers, a 200 lb 10 rep max set would calculate to a 267 lb 1 rep max. 20% over the 10 rep max would be 240 lbs, or 90% of the one rep max. It is not a 1 RM lift, but a 90% of 1RM lift and 60 second concentric, then drop set reps with 120 lbs.
As to Scott’s query, I trust in Dr. Darden to shoot straight, and if he believes Big Jim enough to write about this method, I can at least give him the benefit of the doubt. Try it once, and if after four days you think it is bs, so be it. But to sit behind a keyboard and write that is too tough or you might get hurt, or whatever, without giving it a true trial, what may you be missing?