Pete, just out of curiosity, why would Tom recommend sugar if cold ferment was longer than 2-3 days? It would seem that sugar speeds up the ferment and would help a shorter ferment time, but maybe I'm missing something?
The recipe in question does not call for any sugar. So, over the course of a few days, the yeast is fed from simple sugars that come from the conversion of damaged starch in the flour to forms of sugar that yeast can use (it can only feed off of simple sugars, which sucrose is not). After a couple of days or so, the amount of natural sugars can be low, so sucrose, or ordinary table sugar, is added to the dough to replenish the supply of sugar to the yeast. Sucrose is a complex sugar that has to be broken down into natural sugars for the yeast to use but this takes time. Sucrose works well to provide crust coloration in the short term but it is because of its caramelization effects.