Obviously not 100% but that would be a minority of rides. If you need a car to get to your destination then you either drive or use another car option (like Uber or taxis). Uber is too expensive (and sometimes too slow) to replace other choices for equivalent distances all the time. It's likely used in situations like rain or emergencies or something else. And people who had no options before or lacked last mile reach can now use Uber so of course there's net new riders too.

Uber will add to traffic but traffic always increases anyway. Check any city over any period of time. It's normal. Uber isn't the problem, it's the lack of capacity for a growing population and other viable transportation. For example, look at electric scooters and bikes in big cities that people are using for medium distances instead of cars. That's what happens when there's progress and competition instead of regressive regulation.