Its not $10B worth. If you tried to liquidate, convert into another currency or asset etc there wouldn't be enough buyers at $9500, $1 would be much more likely towards the end of selling...
It is safe to assume that CoinBase has a larger order book, so I think that the slippage could go down to 50% if you wanted to sell $10B worth of BTC, so it would be more like $5B. Sure, that is if you want to sell it in one go, but usually you make a stop order or a limit order to protect you.