Don't know about the parent but it's so legal in SF and the Peninsula that plenty of sidewalk spaces have sharrows (https://en.m.wikipedia.org/wiki/Shared_lane_marking) or signs indicating cyclists are allowed use of the path. These signs aren't necessary for cyclists to use the sidewalk though: we're simply allowed to unless a sign explicitly prohibits it.

Besides the point isn't that cyclists in these spaces shouldn't have to look out for pedestrians, it's that pedestrians make it hard even for other pedestrians to navigate and that perhaps an inequitable share of responsibility for observing one's surroundings exists.

Don't know about the peninsula but biking on the sidewalk is illegal in SF.