The core tenant of the paper is that roughness reduces drag IN the transition zone. A very small region of the total flow.

Thats the region between laminar and turbulent flow. Laminar flow is typically 5x less drag than turbulent, and will be encountered about a Reynolds number of 500K-1M (ratio of inertial flow to viscous flow).

Surfboards will have a Reynolds number of 10^7 which is entirely turbulent.

A Cessna aircraft will have a Reynolds number of 1-5x10^6.

> core tenant

And Lady Mondegreen.