Why disc brakes? It's an expensive way to build a bike (forks are more expensive, calipers are more expensive, wheels are more expensive) and completely unnecessary to slow down very low mass
Because the bamboo bike rear dropouts are designed for it. Assuming that GWaudax is using the kits/parts from Bamboo bike club, they have a clever rear drop out that is designed for disk brakes, and thru axle and QR, and single speed and derailure. Also given the bamboo bike build, it saves having a big load put on a tiny bridge part that may not be positioned with precision.
I'm guessing here, but that would be reasoning.
Oh, Calipers are free, they are a donation, and they allow for multiple wheel sizes, so you can make a frame that uses different wheels as the rider grows. Just as another reason...
J