So could a billionaire buy a million BTCs and try to pump and dump BTC to control the value like they do in the stock market Dark Pool?
Possible, since probably half of Bitcoin are stored in centralized exchanges, so billionaire can buy Bitcoin through CEX without need to move the coins. If both billionaire and CEX didn't publicize if the billionaire bought Bitcoin, no one will know it.
Bitcoin market cap is really huge, it's worth $1 Trillion. But if few billionaires agree to use their money just to buy Bitcoin, they could own more than 50% of Bitcoin.