(Some of) the drivers are advocating for a (California) government policy (law) that would make drivers be treated as employees and therefore require their employers to pay minimum wage while on duty, have regulated minimum and maximum shifts with regulated breaks, etc etc.

Uber is saying don't do that, instead proposing a new government policy (laws or regulation) of a minimum rate of $21/hour while providing a ride. Like minimum wage, this would be paid by the employer/client, but the government sets a price floor.

Note that how this compares to minimum wage for employees depends entirely on the occupancy rate for the vehicle: if you've only got a fare for half of the minutes in a hour, that's $10.50/hour which is under the 2019 California minimum wage of $12 or $11 for small employers.