Also being Silicon Valley, they have more companies than average that were well positioned to wfh. And with high rents, people are more likely to live in a smaller apartment, making wfh even more uncomfortable... making them even more likely to want to leave.
Rent surveys are usually heavily biased towards large buildings. SF has a ton of new development in currently moribund neighborhoods, so I suspect some of the massive drop is due to that being oversampled (Mission Bay, for instance). In the more desirable neighborhoods with lots of rent controlled places the price drops likely aren't as steep. My friend is trying to rent a condo in a desirable location and most people looking at it are moving within San Francisco, trying to get into the more established neighborhoods at a better price.
They don't explain why rents in SF are falling more than anywhere else.
To be fair, the article only says 31% is the most. If other cities are around 29-30%, forget what I said.