It's not so much about rolling resistance but comfort. Aero is great, rolling efficiency is great, but after 200km, if your ass hurts, your hands hurt, etc. you'll be fatigued and putting out less power.
So 28mm measured tires on 29mm wide rims with 21mm bead hooks makes a whole lot of sense. If you're giving up a couple of watts at low yaw angles, it really doesn't even matter that much because for the majority of the race you will be at endurance pace or protected inside a peloton. Wider tires with lower pressures also corner significantly better, which could be key at the end of a stage.