I thought they did have a stop button. I recall a video James May made of a Waymo that had one. I could be wrong. But, the article doesn't say anything about whether one was present and if the occupants tried it.
Edit: I just got home and rewatched the video. No, there's no emergency stop button. There is a "pull over" button on the passenger touchscreen console and the app, but that's about it. A bit concerning!