In this case I think the ice water is not for flakiness or workability. The crust isn’t flaky but it gets very crisp. Rheinhart is very into manipulating time and temperature and that sometimes mean slowing down the fermentation of the bread yeast to make other flavors develop. I think in this case there is more time once the dough is wet for changes to occur in the carbohydrates and gluten without the effects of fermentation. Maybe this helps workability, I suspect it does but don’t know why. It definitely makes for more maillard reaction and complex flavor as well as air pockets. His recipe for Pan L’ancienne is very similar and something I highly recommend if you like a crispy crust, moist silky crumb, and lots of holes.