I would think it is more likely that the batch of coins was in a hopper and the hypo solution was tipped in to achieve the toning. Then, as well as the issue of some coins being tightly packed together, and so getting uniface treatment, there is the issue of the hypo becoming used up as it reacts with the coins, resulting in different levels of toning through the batch. It is possible that all coins went through the process, some ending up heavily toned, some partial and some not at all.
It may also be that the experts in the past had access to Royal Mint records saying all were toned and were repeating that information, regardless of the evidence of different levels of toning apparent in circulation.