That is only the case for call options, which isn't the case where you're concerned about counterparty risk correlation with the underlier. For put option, they'll need to guarantee enough cash (or cash equivalents) on hand to pay you out if Bitcoin reaches zero.
The carrying cost (or at least opportunity cost) is high for holding large inventories of cash, so it's not very attractive from a business perspective to do so unless there's either a market or regulatory demand to do so.