You're forgetting the unoccupied trips. There will be unoccupied trips running errands even at rush hour, because why not, you're not sitting in traffic, just your car.
Also you seem to be thinking more of the robotaxis model, but the article is talking about the more likely future of everyone continuing to own their own car, only now it can drive. If cars are driving back empty after a commute then rush hour will have roughly twice the traffic.
One way to prevent congestion would be to instead of paying for the distance traveled, you simply paid for the time the car is on the road, giving a financial incentive to send the car outside of rush hours. Would probably also cause people to use alternative routes rather than everyone trying to use the shortest one when is congested.
Wouldn't that be a trip that someone would normally be driving anyway? Also wouldn't it make sense to have a store deliver to lots of people on one circuit, this reducing traffic?
What are you basing that on? In dense cities with good transportation infrastructure people don't own cars nearly as much and the people that do own cars use them less.
Also you seem to be thinking more of the robotaxis model, but the article is talking about the more likely future of everyone continuing to own their own car, only now it can drive. If cars are driving back empty after a commute then rush hour will have roughly twice the traffic.