As far as I can tell here the ghost portion is basically a non-sequiter. The waymo vehicle called the cops because it detected e passengers had a gun. The vehicle did not know or care about its status as a ghost gun or not. I'm sure that the Waymo terms of service prohibit carrying weapons into the vehicle. I'm pretty sure that if you were brandishing a realistic looking enough gun, an Uber driver would kick you out…
There's a big difference between a human present who can make a decision based on a situation literally in front of their eyes, and a computer flagging a video feed for someone in what amounts to a call center to look at and decide on. Not to mention the mass surveillance this sort of stuff continues to help enable.
That's what happened here. A private company saw a gun inside their property and called the police.