To answer my own question, competitive cubing uses unbiased randomization algorithms.[0] To minimize scrambling time, it could fairly generate a random configuration and then optimally scramble the cube in ~18 moves.[1]

TL;DR fair scrambling is exactly as fast (same throughout) as solving random cubes! Neat.

[0] https://www.cubelelo.com/blogs/cubing/how-to-scramble-a-rubi...

[1] https://en.wikipedia.org/wiki/Optimal_solutions_for_the_Rubi...