Does it even offer any realistic protection if the "partner bank" goes under? AFAIK the limit on FDIC insurance is $200k / account. Is FTX managing hundreds/thousands of account to get around this or is only $200k of their money FDIC insured?
To get FDIC insurance in a meaningful way it would have to be per-customer accounts created in specific ways.
If they're not created in particular ways the funds in the third party bank accounts can be taken in bankruptcy actions by creditors and aren't actually protected customer funds and aren't insured at all accept with whatever level of insurance they have for the partner bank going under, but that is not the risk anybody is worried about.
