If we are to take the content of your post at face value, that thesis is wrong. It was due to sand hill road being where sand hill road is. That may have something to do with taxes and regulations to a degree, but that isn’t a sufficient explanation.

and within the US, "why SV?" might simply have another orthogonal explanation.

If that were the case, you would have to see unicorns in Delaware and Texas and many other places in the US, especially since many of these have a much more lax tax regime than California or New York.

The fact that unicorns are almost exclusively bred in two cities in the US actually suggests that the influence of the tax and employment regimes is overwhelmingly dwarfed by other factors.