Certainly, there is spare capacity of empty ride-sharing cars; it's not just-in-time inventory where they drop someone off and immediately pick someone up.

Also, in many jurisdictions, capacity for taxis is limited; that's what the medallions are for. Ride-sharing capacity is not limited.

I haven't seen data on whether taxis or ride-sharing have more spare capacity in terms of miles, but I have seen data that ride-sharing vehicles significantly increase traffic.