> 401k accounts are FDIC insured up to $250k

No they aren’t. Possibly your specific 401k could have a cash fund at an FDIC insured bank, but that still wouldn’t be the 401k that is insured, it would be the bank.

SIPC insures brokers up to $500k, which would generally cover most 401k administrators.

You should note, however, that 401k assets do not belong to the administrator anyway, broker insolvency does not effect your 401k in any significant way. SIPC coverage only really comes in if the broker is pulling a Madoff, and stealing the money while pretending to invest it.