Walnut Creek is ~35 miles straight-line from SF, and in a different county. Among other things the bay and the Oakland Hills (and Oakland itself, for that matter) are between SF and Walnut Creek. I-680 never comes much closer to SF than 35 miles.
Like Santa Clara County, Trump got over 25% of the county's vote in 2016, almost triple what he got in SF County. So 30 people with a pro-Trump banner on a freeway overpass is not all that surprising. Nor that there were honks of approval. I don't think California is in play in 2020, but I think he could top 40% of the vote (he got between 32% and 33% of the vote in 2016). I also think it likely that the Ds will lose a few House seats in California.
But I do remember what my guess plus a Hamilton would get you at Starchucks,
.