No, the bonds are worth something, and the losses can narrow by a lot if they're allowed to hold to maturity, deposit interest rates rise slowly enough and if broad interest rates drop some time on the future. The risk that they can't narrow is indeed real and whether to backstop that is a source of contention here.
"slightly less valuable than expected" is not "worthless"! The hyperbole bubble is getting exhausting. People really want to talk a crisis into existence. Are they bored?
There is no $600B of worthless bonds. There is $600B of unrealized losses. The bonds are worth something.
Moreover, unrealized losses are not leverage. Banks in 2008 were levered 30-to-1. No such thing now.