At the root of the issue was that they where not solvent when bonds where valued at market-to-market, but you don't really care about that when you are intending to hold the bonds to maturity. They where solvent when bonds where valued at maturation rates. The latter valuation is what you traditionally calculate and use in banking, the prior only becomes relevant if there is a run on the bank and you have to liquidate immediately to cover, which is of cause what happened.

The great comedy of the situation is that if everyone had been calm and said "hmm, they need to raise some capital lets see what happens" SVB might have had a bad quarter or two, but they wouldn't have gone bankrupt. They only went down because the startup community reaction to hearing "we need to raise capital to cover day to day liquidity requirements" was to initiate an immediate flash run on the bank, which meant they where forced to sell all those bonds at a discounted market-to-market rate instead of holding them to maturity as was the expectation.