Calling bonds a bad gamble is questionable. The Fed rate goes up, the price of bonds at the old rate goes down, a minor bank run forces them to sell the bonds at a loss to cover the withdrawals.
Were they supposed to invest in stocks, options, or real estate instead? The answer is complicated, but calling bonds a "bad gamble" is weird
Protecting banks at all is not part of the Fed’s monetary policy mandate, so the “bag gambles” was surplus verbiage, ultimately.
Price stability and employment are; protecting banks from failure is a non-goal except insofar as it might instrumentally serve the actual dual mandate goals, and there are mechanisms in place to protect the economy from bank failure impacts, and if the people responsible for them (which include the Fed, but in a supporting rather than leading role, abd outside of monetary policy) are on the ball, the impact of such failures on the things that are in the monetary policy mandate are minimal.
They were supposed to invest in short term bonds instead of long term at the very least. If SVB had one year maturity dated bonds instead of 10 years they'd be fine today.
No, it wasn't, and protecting banks from bad gambles isn’t either side of the dual mandate.