Your argument is conclusory. The whole question is the extent to which the regulatory environment is preventing upzoning. If you think San Francisco should remain zoned the way it is, say that, and say why.
I have no opinion one way or another how SF should be zoned. I'm just saying that how it is zoned is a second- or third-order effect on the rate of home-building. Land, labor and materials are the first-order effects. Now, maybe SF is saturated, and every square foot that could have a home on it already has one. Then you'd have to rezone other areas in order to increase the supply of land. But even if you did that, as long as the land was a lot more expensive than Austin's, the rate of home construction would still be slower. And I think it would be physically impossible for SF to zone its way to a land supply comparable to Austin. Hope that clarifies.