I would argue it's not necessarily a problem to have liabilities for which you do not have assets on hand, whether it's Bitcoin or some security. The difference between these two situations is:

1) FTX didn't even bother listing their customer liabilities on their balance sheet, unlike Citadel.

2) I would assume (hope) that Citadel isn't a completely fraudulent company without the capital to make good on those assets if delivery is demanded!