I don't think that's super conclusive. As others have mentioned, Californians are awake and during their workday right now, other timezones may be sleeping, and some might not see the post at all because they tend do read HN less on the weekend, for example.
I would not be surprised at all if California has a big share, but I don't think this poll is a sure enough way to gauge that. It would have been more insightful if California wasn't one of the top answers right now.
So yeah, "Silicon Valley".