This is only true for treasuries which are risk free. Any other bond includes other confounding risk factors impacting price which come out in MTM. These were agency bonds so would include duration/prepayment risks. But you also can face substantial liquidity risks in a fire sale, if you mark your assets to the full liquidation value, you need to mark against an order book to the depth of your holdings. Simple HTM NPV assumes you don’t face these pricing risks because you are holding for par+interest, discounted at risk free rates.