Most people copy what others do so it is mainly a fashion / conforming thing. Panniers are certainly not aerodynamic, but - while plenty of people have weighed them - I don't know that anyone has ever aero-tested a Carradice!
The downside with many bikepacking bags is poor accessability - you have to tip all your stuff out on the road to find something you need. With careful planning of packing you can largely get round that.
One bike has a tailfin rack but I didn't spot any 'normal' racks.
I can't decide if riding a Brompton on an event like this eccentric or disrespectful.