I'm not a lawyer; my exposure to employment law came in a previous existence as a workplace union rep and this is somewhat outside my expertise.

As I understand it, if the Uber driver were allowed to have someone else drive their car for Uber when they felt like it, and had the freedom (actual, not nominal) to decide which clients they'd like to take on, and had control of their pricing, then there would be no question it was actual self-employment.