I'm not in San Francisco, but if we were to experience a doubling of inventory here in Seattle, it would be evidence of a big shift in the market. We neighbor-visited an open house on our block in the autumn that was attended by at least a person a minute during the half-hour that we were there.
The article is super interesting for its incongruence with local anecdotes, but it would also suggest that the "techie exodus" in SF has some roots in reality.