That's the loss that took down the SVB, and it's a real economic loss. If the SVB's depositors behaved like the simplest textbook model, then as soon as short-term Treasury rates increased, they'd insist on correspondingly increased interest on their deposits. The cost of that extra interest would be the SVB's loss--the cash flows in from the bonds stay the same, but the cash flows out to the depositors would need to increase.

In reality, I understand that depositors are generally "stickier" than that, leaving their funds at their existing banks even when bank interest rates increase slower than Treasury rates. That gave the SVB some hope that they could earn their way out of the hole, passing the loss slowly to their depositors over time by paying below-market interest rates. That's the behavior that HTM accounting roughly models. The SVB's depositors had no economically rational incentive to accept that though, and they didn't.