Patrick, the comment I reference is at the bottom of Page 366, right column which describes the experiment in figure 16.21 in detail. "oxidation products.... retard development" and then "activity returned by addition of sodium sulfte".
This is the problem with a non chemist reading an argument on chemical reactions. This is A + B = C where C buildup retards development.
If C has nowhere to go the reaction can virtually cease, but if C + D = something else, the reaction speeds up.
Now, lets write Silver halide + HQ = Quinone + Silver metal. Silver metal is a solid, but quinone is in solution and blocks up the furhther reaction of HQ, all by itself or by making Quinhydrone.
If you have Q + sulfite = HQ monosulfonate, then the reaction speeds back up again. QED.