Earlier quoted context omitted.
I think you're on to something here; usually when you see transactions that seem absurd from the outside, there is some other angle that you don't know about (like, you can rip down the existing house and build one 3x the size that you can sell for $8mm).
Or 6-7 smaller houses and sell them for ~$1m each.
But yeah, that has happened elsewhere in the Bay Area. (Zoning tends to prevent it, though, so there's an element of regulatory arbitrage.)