seems to me that many of these depositors couldn’t really have used such a service anyway. The way it works is that when you deposit, say, $1 million in bank X, the service keeps keeps (a little less than) $250k in the first bank. The next (little less than) $250k goes to banks 2, 3 and 4. (they keep it under 250k so that accrued interest doesn’t push it over 250k). BUT, in return, OTHER banks have to send $750k back to bank X. (Not necessarily from 2, 3 and 4) In other words, you are depositing $1m in bank X, and bank X has to have $1m in deposits - it’s just that $750k comes from other banks in exchange for your $750k. This is all done in computer bookkeeping - money doesn’t actually move.
So if you’re Roku or somebody and want to use this sort of service to spread hundreds of millions of dollars, there has to be enough money in other banks to come back in the other direction. Given the volume, this could have been difficult.