This seems... interesting, legally speaking. I imagine the idea is that you're implicitly accepting alterations to the previous contract by opting to take the car? Would that argument hold water, legally?
My first trip to Paris many years ago, I felt that I was getting the "scenic tour" to my hotel from the airport but I wasn't sure where I was and certainly didn't know the best way between the two. There were other variations in other cities.
For a while, I lived near an airport and taxi drivers would be angry because they'd waited a long time to do a short ride and now would have to go back to the end of the line.
And according to other comments here, the "fixed price" isn't actually true - it appears some ride-share apps will add fees at the conclusion of the ride.
You paid a premium for promised X, specifically. Y showed up. This is the equivalent of buying a first class ticket, and getting put in economy.
Someone should class action this bs.