It's truly fascinating how little the underlying technology of cryptocurrencies matters to the exchange rates and how they're traded.
Fun fact: Ripple actually lost some of the early history of their ledger! Everything before block 32569 was lost.
Yes. That's why Coinbase got out of margin trading. They claim the SEC made them do it. But all the SEC did was to say that "delivery" meant delivery to the customer's wallet, not an entry on Coinbase's books. Coinbase was willing to exit margin trading rather than actually pay out.
This follows the procedure for physical commodities, like grain. A key point there is that the organization storing the grain is totally separate from the trading process. What you're trading is a warehouse receipt for grain in a grain elevator somewhere. You can "take delivery" by getting the warehouse receipt and starting to pay storage charges. At that point your broker no longer has access to the asset. Eventually someone buys it who actually intends to use it, and they have it loaded onto a train or truck.
The key point here is separation between the broker, exchange, and storage facility. Cryptocurrencies use cryptographic wallets instead of warehouses, so "delivery" means delivery to the user's wallet.