What law are you referring to, and a minimum of $20 per hour for what? Wouldn't your interpretation mean I can sign up for DoorDash, keep the app open, never accept a delivery, and be guaranteed $20/hr?
I don't know, but presumably OP is referring to a $20min wage as covering active time, not "dashing" time. The latter means you have the app open and are scheduled to work. Active means you are actively driving to a restaurant, picking up food or dropping it off.
So if you schedule two hours and the app give you no hits, or you decline hits that time wouldn't count.
Now that said, I know nothing about the $20 min wage and can't say if that's accurate or not.
That states they can choose hourly minimum or "or pay per delivery at about 50 cents a minute" which is in line with $29.93 per hour for what DoorDash calls "active" work.