Sure, banks can create money "out of thin air" because the liability created when the loaned money is debited is balanced out by the asset of the loan owned to them.

But if the loan defaults, the asset disappears, and if this happens enough the bank risks becoming insolvent and no longer being able to repay other liabilities like its deposits unless bailed out.

Maybe that's what you're alluding to - the Fed will always bail them out so deposits are never lost? That's probably true in practice, but a loan from the Fed can't in of itself restore solvency, since it's still both an asset and a liability. In the most extreme case, the bank would still go bankrupt, investors would lose their money, and depositors could also lose some of their money unless individually bailed out by the FDIC.

Back to the original discussion - the key question here is are people who hold cryptocurrency at Coinbase considered "investors" or "depositors", and are they protected by the FDIC? The latter is almost certainly no, and the former determines how much they can expect to get repaid if Coinbase goes bankrupt.