a depositor isn't going to deposit 1 billion and only expect 1 billion back out months later. that's some poor people shit. Large sums of money like that, banks pay you for the pleasure of holding it. So a bank offering 4.5% APY like SVB offers would owe an extra 3.2 million to the billionaire if they look at their account a month later.
The problem is, the underlying assets that SVB owns will only pay them back at 1% APY, and only in 20 years or whatever, and the billionaire has been promised 4.5%APY and is expecting to have access to one months worth of interest next month. that 3.5 difference is thus a huge problem for SVB.