(note C vs F on the legend)
As someone who lives in California, and likes snow sports, it looks like I could run into -20C conditions in the Sierras, and certainly the Rockies (although driving that far with an electric car would probably be too annoying to attempt).
It’s probably not a concern for the average Bay Area family.
But that said, zones 6a/6b are definitely in the region of "yeah, I'd worry about 0°F overnight", and people in 7a will definitely worry about temperatures around 10°F if they're going to start their car. 6a-7a covers most of the population corridor in the NE US, about ⅓ the US population. Additionally, much of the Midwest is in the 0°F-is-possible territory, from St. Louis and Chicago straight through to Detroit and Pittsburgh.
The general point is that starting in -20°C isn't some "well, sucks to live in Canada/Minnesota/North Dakota" concern, but rather "gee, a significant chunk of the US population has to do this on an annual basis."