Generally, the FDIC will go over $250k if the bank has the assets for it - the guarantee is $250k, but you could be made mostly whole. People online in the banking sphere are suggesting that SVB accounts may be getting a 20-30% haircut (except those at $250k or under, which are not going to lose anything).

Bill Ackman wants them to insure every dollar held at SVB, but not necessarily make a rule to always do this. He only wants his own bank accounts insured, not other peoples'.

Edit: Clarifying - the FDIC doesn't pay anyone to get to a balance over $250k, but they are the ones who administer the bank when it is in bankruptcy, and they use the bank's assets to cover depositors' interests first. In some bank failures, the FDIC ends up spending almost nothing on the insurance because the assets end up covering the accounts completely.