That's at best a misunderstanding of what that means, and a less charitable reading of your comment would be that it's deliberately misleading. "San Francisco"

That's at best a misunderstanding of what that means, and a less charitable reading of your comment would be that it's deliberately misleading. "San Francisco" refers to the metro area which includes a much larger area. That includes 28K homes that are on the market right now. When people have studied why homes are vacant, another substantial fraction of it is homes being remodeled or otherwise uninhabitable. That's very much NOT 100,000 livable houses sitting around being owned by speculators. A much better way to look at it is vacancy rate. SF's vacancy rate is around 5%, which is very low and indicates that nearly all of the housing stock is being used. For a lot of the same reasons that a 0% unemployment rate is not possible, it's not possible to have a 0% vacancy rate, and in practice 5% is about as low as you can get.