It isn't hard to dump those funds into a money market fund backed by short-term commercial paper or even short-term Treasury bills. Or to just buy the Treasury bills outright. Such holdings are quite liquid and can be absolutely secure.
Use the bank account for clearing, keep a couple million in it and sell assets as needed to top-up the account or to prepare for known cash outflows.
I'm sure there are cost and complexity trade-offs. But "don't lose the cash" would seem to be priority #1 and worth some trouble.
I suppose the idea was that SVB managed all that for you. But one look at its financials shows the asset/liability term mismatch, and interest rate risk, so the risk of loss of cash was nonzero. So they were NOT managing maturity risk for these large depositors, and, well, now look where they are.