FTX CEO litterally said that in his tweet.
Banks make similar statements all the time -- they require regular audits of assets (stress tests) and maintain some minimum levels of liquidity.
Note: There's a difference between being a custodian of customers' investments (brokerage / commercial banking) versus proactively investing customer deposits (investment banking).
From https://en.wikipedia.org/wiki/Fractional-reserve_banking:
"Fractional-reserve banking predates the existence of governmental monetary authorities"
Real regulated banks have access to the Fed to borrow in a case of a bank run.
Anyone who has more than the insured amount, or anyone who needs their money in the near future will still engage in a run.
There were several runs during the 2008 crisis, the most famous is the run on IndyMac Bank.
Shadow banks don't.
"We never block withdrawls" is the new "We don't crash ever" as seen in the Facebook movie.
If you are in the crypto exchange business you gotta do both actually. Don't crash the website and don't suspend withdrawls ever.
Short dated government bonds would be the safest. But tweet 0 he litterally says they dont do that, they keep the cash under the mattress and you can have it if you stop by. People stopped by and there was no cash under the mattress...
It is actually irresponsible to the users (risk management wise) to be keeping all that cash in hand 24/7. The easiest bad case example: are you keeping all your savings under your mattress?