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.
