Cars do not use ultrasonic sensors for autonomy. They are for low-speed situations (such as parking) and are designed to give more information than the driver is otherwise capable of gathering.

While I generally agree to get parity you would use common sensory input, I would hope the goal is to go beyond human level and navigate in for e.g. fog conditions or heavy rain, etc. things radar would help with. Ultrasonics probably don’t buy them much at this point as long as they have good object permanence. Think about parking in a garage. If you can map objects to special memory adequately then maybe ultrasonics aren’t necessary. However if the object is out of sight out of mind, you probably need the ultrasonics to help not hit it.