Can confirm, just spent 30 summer days in Florida. People don't pull over and stop, they just put on their "hazards" and slow down, sometimes to 25-30 MPH depending on the intensity of rain. This happens every single day.
I've seen people do all sorts of foolish things. When the rain is so hard that I can't see more than 15 feet, I pull over and stop.
Self-driving cars would be putting lives at risk if they just pulled over and shutdown on every summer thunderstorm.
It's certainly not an unanswerable question. It's just quite hard to verify that they answer you have is a sufficiently correct one.
Why? It seems to me that estimating the confidence of a prediction by the quality of measurements is not an infrequent machine learning task. It'd be more complex for a driving algorithm than calculating the confidence interval of a linear regression, but it's in the same theme.
Humans can and do drive safely in these conditions, however, and it's a valid concern to point out that Waymo's West Coast-centric models might not yet be trained to handle these cases.
(To be clear, I don't doubt that machines will eventually surpass humans in whatever weather conditions, if they haven't already.)