Where does this belief come from? It's commonly held among Bitcoin investors, but it makes no sense to me. Why would any investment be immune to real world events?

Bitcoin investors are real people and real institutions. They are affected by market panics and economic downturns just like stock and commodity investors are. At best, we could hope that bitcoins hold value better than stocks do during an economic crash. Gold seems to fit that niche, but it certainly isn't "immune" to the world economy.

I don't like commodities as safe haven assets. In a crash, demand for, and consumption of commodities tends to fall. Perhaps their prices fall less than stocks in this situation, but stable fiat currencies seem like a much safer financial hedge.